When Japanese artist Megumi Igarashi began to make plaster casts of her genitalia and show them in exhibitions, she accepted some people might be upset. But she could not have imagined she would end up in prison.

Ms Igarashi has spent 10 days in a Tokyo women’s prison. Her crime, the police say, was sending an email to a “large number of people” with a link showing them a plan for how to create a boat using an “obscene” 3D image of her vagina.

It’s the second arrest of the artist, who uses the pseudonym Rokudenashiko, roughly meaning “good-for-nothing girl”. In July she was imprisoned for six days after she tried to use crowdfunding to pay for a kayak in the shape of her genitals.

Supporters say the arrests reek of double standards in a country where sexualised images of women are rampant and where consuming child pornography is legal until next year.

Igarashi says she is trying to demystify the vagina. “Woman have very little opportunity to see their genitals outright and know very little about their own bodies,” she told reporters after her first arrest.

Nothing obscene

“I have learned to love my whole body. To me, my vagina is like my arms and legs. It’s nothing obscene.”

Japan’s obscenity laws prescribe punishments for images that “wantonly stimulate or arouse sexual desire, offend the normal sense of sexual modesty of ordinary persons, or are contrary to the proprieties of sexual morality”.

Igarashi’s harassment has raised eyebrows because that seems a perfect description of much of the output produced by Japan’s famously lively porn industry. Convenience stores stock magazines containing images of incest, underage sex or gang rapes.

Mass circulation magazines publish photo stories showing young girls in various states of sexual distress.

Japan seems less inhibited by images of the male organ, points out Igarashi. A popular annual fertility festival near Tokyo features giant wooden phalluses and penis-shaped candy.

“The vagina has been long regarded as obscene while the penis is considered pop,” she says.

“Japan is completely inconsistent. If you ride the train you’re confronted with advertisements with sexual images and you cannot avoid looking at them. All these notions are based on the men’s viewpoint and there is a lack of empowerment of women.”

Her lawyer, Takashi Yamaguchi, says he has no idea why Igarashi has been singled out. But he says Japanese judges have "very broad discretion" to determine what's obscene. "This case is about what are the prevailing social ideas and what constitutes normal sex."

Tokyo police also arrested the owner of a sex shop where Igarashi exhibits her work. A police spokesman told Reuters that “obscene figurines” had been displayed in a glass case where visitors could see it. “This is a new factor,” he said.

More conservative

The artist says Japan is becoming more conservative under prime minister Shinzo Abe, despite his repeated pledges to empower women and improve the country's dismal gender ratings. Japan came 104th in the latest Global Gender Gap survey.

The case has triggered a fierce debate about censorship, women’s rights and what constitutes obscenity.

Igarashi’s supporters have collected thousands of names in an online petition to free her.

If charged and convicted, she faces up to two years in prison.