Millar praised the 70,000-capacity International Stadium Yokohama in the western outskirts of Tokyo, which staged the 2002 soccer World Cup final, on a three-day visit to the country.
Japan, which co-hosted the 2002 soccer World Cup with South Korea, wants to bring rugby's premier competition to a country outside the sport's traditional venues.
It is bidding against New Zealand and South Africa for the right to bring the World Cup to Asia for the first time.
The 2011 host will be chosen in November.
