Whether the Bai-u front is polar front or not is very interesting problem. Some climatologists and meteorologists have considered the Bai-u front as a part of the Pacific Polar front, and the others have thought it the subtropical front. To solve this question, it is necessary to investigate the relations between the Bai-u front and the Polar air mass considering the seasonal change of the Polar air mass. In this report, the author has examined above problem. The results are summarized as follows;<br> 1. According to the examination in the continuity and the latitudinal fluctuation of the surface fronts along 140 E, the Bai-u season in 1985 was divided into four stages (Fig. 3).<br> 2. The analyses of wind shear and equivalent potential temperature showed that the Bai-u front was maintained by the subtropical jet stream (Fig. 4, 6). There was the highly mixing air formed by heating from the surface of the earth on the north of the Bai-u front (Fig. 5, 7). The polar front accompanied with the polar jet stream was seen in the north of the Bai-u front. In the following paragraphs, the author explains the relations between the Bai-u front and the Polar air mass in each stage.<br> 3. Early in May (Fig. 18-a)<br> The Polar air mass uniformly spread over the continent and the ocean of East Asia. The polar front was formed around the southern limit of the Polar air mass. The Bai-u front is different from the polar front because the Bai-u front is not formed in this stage.<br> 4. Early Bai-u (Fig. 18-b)<br> On the continent, the Polar air mass was reduced at the north side of 60 N because of the heating from the ground. While on the ocean, the volume of Polar air mass was large and its southern end reached the Pacific coast of North Honshu. The southern limit of Polar air mass does not reach the Bai-u front. Namely, the Bai-u front is not found as the southern limit of the Polar air mass.<br> 5. Late Bai-u (Fig. 18-c)<br> The center of Polar air mass shifted to the Arctic region and the upper limit of Polar air mass around the Baring Sea and the Sea of Okhotsk was at the level of about 900 mb (1000 m). The occlusions of fronts were observed frequently at far east of 160 E through the Bai-u season. Therefore the fronts of this area may be thought a part of the Pacific Polar front.<br> 6. Late July (Fig. 18-d)<br> The Polar air mass more reduced its volume at the area on the Bering Sea and the Sea of Okhotsk than in the late Bai-u season. The highly mixing air was not observed on the continent.