Among the followers is a 33-year-old man from Tokyo who works weekdays as a sales representative. On his days off, he heads for the mountains, donning a traditional outfit, complete with a conch-shell horn and straw sandals.
He is a yamabushi, a mountain priest trainee. His grueling training regime includes a discipline called nyubu, which involves walking steep mountain paths for a few days while visiting sacred sites and worshipping gods and Buddha. He has a religious name: Shinanobo Zuiryu.
Shinanobo belongs to a group called Nikko-Shugendo in Tochigi Prefecture.
Through strict training, Shugendo followers try to experience what Gautama Siddhartha underwent before attaining enlightenment.
Trainees are called yamabushi or shugenja and undergo various types of training.
Shinanobo first became interested in "mountain religion" while studying history in college. As he deepened his study, he ended up becoming a yamabushi at Nikko-Shugendo.
In 2001, he became a full-fledged member of Nikko-Shugendo. He now uses his days off from work to further his training 10 to 15 times a year on Mount Nantaisan in Nikko, Tochigi Prefecture, as well as other mountains.
Why does he devote himself to training despite his busy work schedule?
"It's my faith," he said.
His wife, Hajime, 30, said, "Whenever he is stressed out in daily life, he says, 'I want to go to the mountains.'"
Hajime is an artist, and she will soon release a work titled "My husband is a yamabushi" in a monthly comic magazine named "Honto ni Atta Waraeru Hanashi" (Funny stories that happened in real life).
Shinanobo is not the only urban resident who longs to spend his spare time undergoing religious training in mountain locations.
Iyano Jiho, 51, who heads Nikko-Shugendo, says those who come from big cities to attend the group's training sessions have a yearning for "mountains."
"For urbanites with little connection with other people and nature, Shugendo training might be offering opportunities to re-examine various involvement with human beings and nature," Iyano said.
Originally from Kanuma, Tochigi Prefecture, Iyano learned Shugendo at temples in Yamagata, Kyoto and Shiga prefectures and elsewhere.
He eventually thought of bringing new life to nyubu training in Nikko, near his hometown. He renovated mountain paths and accommodations for trainees, which had not been used for a long time. The renovation work was completed in 1985, and nyubu training resumed in Nikko.
Another yamabushi from Tokyo is a 30-year-old contract worker, whose religious name is Yamaguchi Horyu. He has been training at Nikko-Shugendo for the past eight years.
As a teenager, Yamaguchi felt strongly that he did not fit in at school. Wanting to "overcome a sense of alienation," he visited several religious organizations, but none of them inspired him.
"They were out of touch with everyday life, and lacked culture or history," Yamaguchi said.
When he was in college, he saw an ad for Nikko-Shugendo training in a magazine and attended a training session. He said to himself, "This is it."
The organization felt "down to earth," he said.
"Myself wearing a necktie and myself in yamabushi outfit are no different, in that I'm a trainee," Yamaguchi said.
Training which involves continued dialogue with gods and Buddha is challenging, he added.
"Still, for me, navigating through life in a big city may be more grueling," he added.
Yamaguchi's fellow trainee under Iyano is a 33-year-old man whose religious name is Kinuki Yuho. He has been training for seven years.
Since his childhood, he had harbored an interest in Buddhism because of his father and grandfather, who were pious.
Contemplating what to do with his life while studying to enter a university, he decided to put Buddhist ideas into practice through his work.
Kinuki got a job as a caregiver but left it after three years to study Buddhism.
While learning about the religion at a university, he met a person training in Shugendo under Iyano.
Asked to come along, Kinuki attended a session and was attracted to Nikko-Shugendo. He became a frequent visitor.
"For instance, in discussing Shugendo teachings in plain language, Iyano said, 'Don't do anything that weighs on your mind, whether it's good or bad.' I repeat these words in my mind, which makes me think," Kinuki said.
Although the words may appear abstract, for Kinuki, it is precious teaching he can apply to real life.
This month, Kinuki returns to his work as a carer.
"I do have apprehension, but I also feel like I have room to breathe somewhere within myself," he said.
"By experiencing the same hard training that trainees ahead of me underwent, such as walking on mountain paths trodden by many others, I feel as if I were spiritually connected with past trainees, as well as with current trainees, beyond time and space. I feel as if I were being encouraged by them. This kind of sense has taken root in me," Kinuki said.
Why do these city dwellers find common ground with Shugendo?
Susumu Shimazono, a professor of religion at the University of Tokyo, has this to say: "Since the 1970s, Qigong, meditation and other spiritual activities have become global trends. However, disappointment is spreading among some of the people who experienced these things. They say things like, 'They are not firmly rooted in society.'
"On the other hand, with traditional aspects as well as physical aspects, Shugendo probably appeals to such people."