The Japanese are paired with both France and the All Blacks in World Cup Pool A, and come up against the former in their tournament opener at Eden Park on Saturday night.
That match is followed six days later by a daunting encounter with the All Blacks in Hamilton, possibly made even tougher after Hirashima backed France's scrum as the one to beat.
"They are the best in the world," Hirashima said. "Stronger than New Zealand's.
Japan have not been backward in coming forward since their arrival in New Zealand, with wing Takehisa Usuzuki speaking about "making history" by overcoming France and moving towards a highly unlikely quarter-final berth.
That was in contrast to former All Black great and now Japan coach John Kirwan, who said they were targeting Tonga and Canada later in the pool stages to gain credibility out of the World Cup.
"We play the fourth and first ranked teams in the world and for us it's about being competitive and Tonga and Canada are the games that are really important for us," Kirwan said. "We've had a great lead-in and a great preparation, but it means nothing and we are judged by the World Cup.
"At the end of the tournament we want people to say that Japanese rugby is the most improved rugby in the world.
"We have a World Cup in 2019 and in 2015 we want to make the play-offs and in 2011 we want to win two games so that we can automatically qualify for 2015."
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