Minami Minegishi of AKB48 appears in tearful mea culpa on YouTube after breaking her band's strict rules on dating

This article is more than 7 years old

This article is more than 7 years old

As pop star misdemeanours go, Minami Minegishi's was tame in the extreme – breaking her group's strict dating ban to spend a night with her boyfriend.

Yet hours after a magazine published photographs of her leaving his home last month, Minegishi, a member of the wildly popular girl band AKB48, went on to YouTube to issue a tearful apology.

"As a senior member of the group, it is my responsibility to be a role model for younger members," she said, before ending the four-minute mea culpa with a deep, lingering bow.

The most striking thing about her apology, however, was her appearance. She had shaved her head, a traditional act of contrition in Japan, but perhaps a step too far for a 20-year-old woman whose "crime" was to have found herself a boyfriend – 19-year-old Alan Shirahama, a dancer in a boyband.

Minegishi explained she had decided to cut off her long hair immediately after seeing her photograph, her face hidden behind a surgical mask and a baseball cap, in the weekly tabloid Shukan Bunshun on Thursday.

Her dramatic gesture underlined the strict rules to which Japan's young pop stars must adhere to project an image of unimpeachable morals.

In the YouTube video, which has been viewed more than 3m times, Minegishi said the assignation has been "thoughtless and immature".

"If it is possible, I wish from the bottom of my heart to stay in the band," she said. "Everything I did is entirely my fault. I am so sorry.

"I don't believe just doing this means I can be forgiven for what I did, but the first thing I thought was that I don't want to quit AKB48."

AKB48's management agency demoted her to "trainee level" starting on Friday, according to the band's official blog.

Minegishi is among the original members of the band, which since it formed in 2005 has built up a huge following among teenage girls and salarymen who flock to concerts and publicity events for the chance to shake hands with their idols.

The group, named after the geeky Akihabara district of Tokyo, has had a succession of million-selling singles and generated sales of more than £126m in CD and DVD sales in 2011.

Together, the AKB48 stable has more than 230 members in their teens and early 20s, with four main bands and several offshoots. Their act is more saccharine than sexual, but the band has courted controversy off-stage in the past year.

In 2012, a TV commercial in which the girls seductively passed confectionery from mouth to mouth attracted complaints, and earlier the same year one of the singer's mothers was arrested for allegedly having sex with a minor.

Last month, police launched an investigation after a weekly magazine ran a photo of a young boy cupping the breasts of the former AKB48 member Tomomi Kasai. The publisher was forced to destroy about 670,000 copies of the magazine.