Japanese prosecutors have charged a feminist artist who makes objects shaped like her own vagina with distributing "obscene" data, in a case that has sparked accusations that authorities are out of touch.

The charges follow Megumi Igarashi's arrest in December after she raised funds online to pay for a genital-shaped kayak which she made on a 3D printer.

"We don't agree with the prosecutors' contention at all," Takeshi Sumi, one of Igarashi's lawyers, told AFP on Wednesday.

"We will continue pleading not guilty on behalf of Igarashi, who argues her works are not anything obscene," Sumi said.

Igarashi, 42, was first arrested in July, but was freed after several days following a legal appeal and after thousands of people signed a petition demanding her release.

But Tokyo police arrested her again in December - along with sex shop owner Minori Watanabe, 44, also a writer and feminist activist, for "displaying (Igarashi's) obscene goods in her shop window".

Watanabe, whose boutique is aimed at women, was later freed after prosecutors failed to persuade a judge to sanction extended questioning.

But the Japanese court approved Igarashi's detention, as prosecutors said they feared she would destroy evidence if released, according to Sumi.

The current charges relate to three counts of distributing "obscene" data - namely CD-ROMs containing computer code for a 3D printer that would allow users to make copies of the vagina-shaped kayak.

Japan's multi-billion-dollar pornography industry is large and varied, but obscenity laws still ban pictures of actual genitalia, which normally appear pixellated or behind black spots.

If convicted of distributing or possessing obscene materials for the purpose of selling, Igarashi could face up to two years in jail and/or a fine of as much as Y2.5 million ($A26,387).