'Invisible' stamp that marks sexual assault attackers sells out in 30 minutes

A 'Women Only' carriage in Tokyo: Groping on trains has been a problem in Japan for decades. Photo / Getty Images

In Japan, a gadget the size of a lipstick has sold out in 30 minutes.

The product has prompted a frenzied reaction as many hope it might help deter a problem that has plagued Japan for decades.

This week Japanese stamp maker Shachihata launched an invisible stamp specifically designed so that groping victims can mark their assailants, reports News.com.au.

The colourless ink is only visible when looked at under an ultraviolet light.

Sexual harassment on public transport has been an issue in Japan for years, with many women reporting being groped on crowded trains.

On Tuesday 500 of the stamps, which cost 2500 yen ($NZ37) went on sale and sold out in just 30 minutes, a Shachihata spokesman told CNN.

Forget Ooshies, this is the item most Japanese women want to get their hands on. Photo / Supplied

"I was so surprised how quickly they were sold out," company spokesman Fumihiro Mukai said.

In May, Shachihata, a well-known Japanese brand, had announced it would be developing a product to help deter groping on trains, known as "chikan".

In 2017, Tokyo police received nearly 900 reports of groping and harassment on their train system.

The figures are despite the fact that those caught inappropriately touching on trains face police fines of up to 500,000 yen ($NZ7,400) and signs warning against "chikan" feature in most major Japanese stations.

In the early 2000s, the Tokyo Metropolitan Police and the JR Railway Company conducted a survey which found two thirds of women aged between 20-40 reported being inappropriately touched on trains.

Women can mark gropers with invisible ink, which can be seen under ultraviolet light. Photo / Supplied

In response, women-only train carriages were introduced, prohibiting men from entering them during rush hour.

The invisible ink stamp isn't the only anti-harassment product made to specifically tackle train groping in Japan.

In 2016 phone app Digi Police launched, with users able to activate a loud voice that says "Stop it" or bring up a message alerting other passengers that there is an assailant in their midst.

Since it was launched, Digi Police has been downloaded more than 237,000 times, according to the Guardian.

"Thanks to its popularity, the number (of downloads) is increasing by about 10,000 every month," police official Keiko Toyamine told the publication.