A high environmental dose rate observed at Rokkasho-mura on December 17, 2015, which was reported to be caused by the wet deposition of radon decay products, was analyzed to depict its spatial distribution and to understand mechanisms of the unprecedentedly high dose rate increase of 0.151 µGy/h. It was pointed out that the high dose rate was observed widely on the Sea of Japan side of the main island of Japan, with areas of slight increases where the precipitation was light. It was also pointed out that the large increases corresponded with the precipitation intensity exceeding about 3 mm/h. However, precipitation events exceeding the value did not necessarily correspond to large increases of the dose rate. According to the analysis using a long-range atmospheric transport model and weather charts, the causal 222Rn was transported from the northeastern part of China and the southeastern part of Siberia, where existed a cold low pressure system transporting 222Rn vertically to form a 1 to 2 km-deep high concentration air layer. This air layer was transported by the winter monsoon to Japan without significant decay of 222Rn. It was found in this analysis that the large dose rate increase was observed at the locations where the significant level of precipitation occurred when the air layer existed.