A Japanese woman who was fined $100 by Chicago Police  for exposing her breasts at North Avenue Beach on national “Go Topless Day” has filed a federal lawsuit against the city.

Sonoko Tagami alleges Chicago’s public decency ordinance violates the 1st and 14th amendment of the U.S. Constitution because it allows men to bare their breasts in public but denies women that same right.

Tagami, 41, was ticketed by a Chicago cop on August 24 for exposing her breasts during a protest at the downtown beach. A YouTube video of the incident shows her topless but wearing a skirt, quietly speaking to two female officers on the beach.

“I say you have to be dressed!” one of the cops tells her in the video. Tagami eventually pulls up her skirt to cover up her nipples. Bystanders than get in a debate with the officers as to whether “pasties” qualify as clothing.

She was ordered to pay the $100 fine and $50 in costs at an administrative law hearing last month.

Her attorney, Ken Flaxman, said Tagami has “a serious lawsuit — she was expressing her political views about gender equality.”

Tagami was living in Chicago at the time of the protest but has since returned to Japan, Flaxman said.

“Go Topless Day” is organized by the Raelian movement — a UFO religion formed in the 1970s by a French former journalist and race car driver. It isn’t clear if Tagami is associated with the group, the lawyer added.

The movement’s website urges women to go topless and states that “women are commonly arrested, fined and humiliated for daring to go topless in public, a freedom men have had for decades.”

Chicago law does allow breastfeeding women to show their breasts in public, but a city ordinance states that anyone whose ”genitals . . . or any portion of the breast at or below the upper edge of the areola thereof any female person, is exposed to public view or is not covered by an opaque covering” is liable for a fine of up to $500.

Tagami was wearing “opaque paint” on her breasts, Flaxman said, though the YouTube video does not make that clear.

“I don’t know about that,” Flaxman said. “I’m just her lawyer.”