Member of AKB48 Minami Minegishi has shaved her own head as punishment for her romantic encounter with a man.

FANS leapt to the defence of a Japanese starlet after she shaved her head and issued a tearful YouTube apology in an act of "disgusting" penance for a night with a man.

Pictures of the roughly-shorn head of Minami Minegishi, 20, were emblazoned on national newspapers and Japan's Twitter scene was abuzz over news that the pop princess from cash-generating juggernaut AKB48 had broken the band's cardinal rule: No Dating.

A sobbing Minegishi told fans she had decided to shave her head in contrition after a popular weekly magazine published claims of a night of passion with a 19-year-old boy band member.

"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," she says in the video, which had been viewed on YouTube more than three million times.

Minegishi, who had long, silky hair at the time, was snapped leaving the apartment of Alan Shirahama, a dancer in an off-shoot of the popular boy band Exile.

Tabloid magazine Shukan Bunshun published its article Thursday, and hours later Minegishi was pleading to be allowed to remain with AKB48, one of the world's most successful acts by revenue.

The tryst was "thoughtless and immature", she told fans.

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

AKB48, a 90-strong pool of girls in their teens and early 20s, is a money-printing machine that makes much of the accessibility - and the implied availability - of its idols.

Fans have frequent opportunities to meet their favourites, who are rotated in and out of the public eye, according to popularity.

In return for their chance to grace television screens, subway adverts and the covers of Japan's countless celebrity magazines, members of the collective must adhere to strict rules of behaviour.

They are allowed to have "one-sided romantic feelings" for a boy but can never progress beyond hinting at their crush - and must never disabuse their legions of male fans that they might one day stand a chance with their fantasy woman.

AKB48's management office said Minegishi will be demoted to a trainee team as punishment "for causing a nuisance to the fans", with the demotion effective from Friday.

But many of those fans took to Twitter to defend the star, saying there was nothing wrong with an adult woman leading a "normal life".

Author and critic Hiroki Azuma said it was "disgusting" that Minegishi had been pressured to the extent where she had to resort to medieval means to make amends for a perfectly normal romance.

User @Shige_Onz demanded to know: "What's the point of this public execution show? It's like something from the war or a totalitarian state."

In Japan, shaving one's head has long been a public display of contrition, although is usually only done by men or boys.

In June last year, another AKB48 member, Rino Sashihara, 20, made a pained apology after a magazine article revealed her free-wheeling history with a boy. She was demoted to a regional sub-group as a punishment.

While much is made of their supposed innocence, the band is heavily marketed on its sex appeal, with skimpy outfits and coquettish smiles the order of the day.

A television commercial in which the girls seductively passed candy from mouth to mouth was a huge hit, despite - or because - of its homoerotic overtones, which sparked complaints from more conservative sections of society.

Most recently, a photograph that showed naked former AKB48 member Tomomi Kasai with a young, apparently Caucasian or Eurasian child cupping her breasts, set off police probe and led to the pulping of about 670,000 copies of a popular weekly magazine.