Sexual assault victims for the first time will also include men, with the definition of "rape" dramatically widened. (Representational/File)

Tokyo: Japan hiked minimum jail terms for rapists and widened the definition of sexual assault victims to include men for the first time on Friday as part of sweeping revisions to century-old sex crime legislation.

Lawmakers decided unanimously to update the 1907 statute to impose tougher penalties on sex attackers and make prosecutions easier, as they look to boost Japan's low number of convictions.

The changes long called for by victim advocates raise the minimum prison term for those convicted of rape to five years, up from the present three.

The changes also means victims will no longer have to file a complaint before a sexual assault prosecution can go ahead. That current requirement is seen as a major reason for the under reporting of rape cases in Japan.

Sexual assault victims for the first time will also include men, with the definition of "rape" dramatically widened. The law will also make it easier to prosecute parents and guardians suspected of abusing their children.

"I hope that these revisions will mean more appropriate punishment and draw public attention to the reality of sex crimes," Jun Yamamoto, a 43-year-old who was molested by her father as a child, reportedly told parliament.

Victims and their supporters said that the current 110-year-old law is out of date and that penalties are too lenient. The number of rape cases in Japan recognised by police declined to some 1,200 in 2015 from more than 2,000 a decade ago, according to police.

But data from a Justice ministry study group found that nearly 75 percent of sex crime victims do not seek criminal punishment for perpetrators.

Support groups said victims tend to hesitate to take legal action as they are traumatised and feel embarrassed to speak out.