The case is a rare example of a public allegation of sexual assault in a country where government data shows just four percent of rape victims report the crime to police.

Rina Gonoi, 22, alleges that she was assaulted by multiple male colleagues while on a training exercise in 2021, a year after she joined the country's Ground Self-Defense Forces (SFS).

She made her allegations public after prosecutors dropped their formal investigation into the case on grounds of insufficient evidence.

"There are many other people, including former senior female colleagues, who have faced sexual harassment," she told reporters after a brief meeting with defence ministry officials in Tokyo to submit the petition.

"I thought nothing will ever change if someone doesn't stand up and take action."

Jiro Kimura, the parliamentary vice-minister of defence, who received the petition, said the government was looking into Gonoi's claims.

"The defence ministry and the Self-Defense Forces take a strong stance that sexual harassment can never happen," he said.

Gonoi said she was offered no guarantees of an apology from the ministry because her allegations are still under investigation.

- Flood of allegations -

Gonoi joined the Ground Self-Defense Forces in April 2020, inspired by a childhood experience of seeing troops help residents of her native Miyagi after the March 2011 tsunami.

A year into her service, she was assigned to a group on a month-long training exercise in the mountains.

She alleges she was called over by male colleagues who were drinking, and two of her superiors, who were talking about martial arts, instructed their subordinates to use a technique on Gonoi.

She says one of the men pressed on her neck, pushed her to the ground, forcibly spread her legs and repeatedly pressed his crotch against her. Two others then did the same.

After Gonoi reported the incident to an SDF sexual harassment counsellor, the case was investigated and sent to prosecutors on suspicion of indecent assault.

But the case was dropped on grounds of insufficient evidence earlier this year, local media reported.

She then began an online petition, demanding "a fair investigation, punishment, and apology", which had more than 106,000 signatures by Wednesday.

It also prompted a flood of more than 100 allegations of sexual harassment and bullying in Japan's armed forces from women and men, which Gonoi submitted to the ministry on Wednesday along with her petition.

Japanese women have access to high standards of education and are well-represented in the workforce, but the country ranks consistently low in the World Economic Forum's Global Gender Gap report.

That is based largely on the dearth of female politicians and women in the upper echelons of business, as well as the wage gap between male and female workers.

The #MeToo movement was more muted in Japan than in some other developed countries, despite a high-profile case in which a woman journalist accused a former TV reporter of rape.