Before European contact in the 16th century, Japanese intellectuals subscribed to the Confucian model that the Earth was a square and the heaven was a sphere. The idea that the Earth was round wasn't fully accepted until the 17th century, in part due to the anti-Christian isolationist stance of the Tokugawa shogunate. However, Japan wasn't fully cut off from the West, as they maintained trade with the Dutch, so other discoveries such as the heliocentric model were gradually introduced and accepted by Japanese scholars throughout the Tokugawa period.

The idea of the round Earth was more easily accepted by the navigators of the red seal ships, which were vessels authorized by the early Tokugawa government (1600-1635) to carry out trade in East and Southeast Asia. The navigators made use latitude and longitude measurements introduced by the West, which necessitated an understanding that the Earth was round.

Source: Hirose, Hideo. The European Influence on Japanese Astronomy.

Although Japanese people have a strong interest in the modern natural science and produce good results on many branches of the natural science (19 Nobel prize winners on natural science), they are terribly weak at archaic cosmology. Before modern times the archaic cosmology was inseparably related with horoscope methods. Japan introduced the Chinese horoscope theories with the method of calculating the (Chinese style) calendar in 8th century, but they could not improve either horoscope or calendar editing until contacting Western astronomy in 18th century. “Ommyoji”? No kidding. Their method was only a poor imitation of Chinese. Before modern times the lack of understanding the archaic cosmology unavoidably meant the lack of an interest in the movement of the planets in the sky and the real shape of the globe. Japanese people had not cared if Earth was round or flat until they knew the Western science.