Airlines in Japan have increased the capability for conveying air passengers by enlarging aircraft size due to the lack of runway capacity especially in Haneda, and consequently the average aircraft-size has been extraordinarily large in Japanese domestic market in comparison with the other western country. This means that level of service in terms of the flight frequency is low in many air routes in Japan. In this study, first, we roughly compare the flight frequency and aircraft size among major countries. Second, in order to see the potential flight frequency, we analyze the flight frequency in US and Japan from the view point of the characteristics of each air route such as passenger demand, route distance and number of airline entries.