"I honestly don't believe the Japanese are aware as to how big an event this is. And that's something we need to push a lot more," Gosper was quoted as saying by Kyodo news agency on Tuesday.
With 406,000 international visitors, the 2015 World Cup in England generated 2.3 billion pounds ($2.86 billion) in economic output, making it the most successful event in the sport's history.
"I think the Japanese government will be surprised at how much Rugby World Cup will contribute to the economy. There's no capital investment involved," Gosper added.
"In terms of impact on the general economy, we saw the Rugby World Cup was bigger than the Sydney Olympics and we saw similar stats for London."
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Despite the concerns, Gosper said he was confident Japan, the first Asian country to host the Rugby World Cup, will have everything in place by 2019.
"Our belief is Japan is ahead of England 2015 in terms of preparation," Gosper said.
"That doesn't mean they don't have a lot to do but we are genuinely confident that everything will be in place as it should be."
($1 = 0.8042 pounds)
(Reporting by Shravanth Vijayakumar in Bengaluru, editing by Ed Osmond)
