An IRB delegation spent two days evaluating Japan's plans to stage the World Cup after visiting the country's seven rivals for the tournament.
"We shared very positive meetings with the JRFU and the Japanese Government and we are impressed with the high level of support from the government," said IRB chairman Bernard Lapasset.
Aso has pledged his commitment to the country's bid by joining Japan's bid committee in the role of executive adviser.
The host nations for the 2015 and 2019 World Cups will be announced by the IRB at the end of July.
Japanese officials say the global economic downturn will not have any negative effect on their bid to become Asia's first World Cup hosts.
Japan lost out to New Zealand in the race to host the 2011 World Cup and faces competition from England, South Africa, Italy and Australia in the bidding for the 2015 tournament.
Wales, Scotland and Ireland have also expressed an interest in staging the event.