The purpose of this study is evaluation of possible petroleum source rocks in the Neogene Tsugaru Basin. Evaluation is based on three criteria, namely organic richness as expressed by organic carbon content, organic maturity as determined by Thermal Alteration Index, and organic type as expressed by visual kerogen composition. One hundred forty-one samples of mudstone from 7 surface routes on land were analysed regarding the three criteria. The results show two characteristic points as follows; (1) Samples from the Middle Miocene Kodomari Formation upward generally include more than 0.5% organic carbon and amorphous type organic matter. But in maturation, they do not exceed the threshold of intense oil generation. (2) Samples of the Lower Miocene Odose and Isomatsu Formations reach the threshold of intense oil generation. But they fail to meet both the organic richness criterion and the organic type criterion. Although it is determined that no source rock was found, this study suggests that possible source rock may be found in two areas, off the west coast and in the east part of the Tsugaru Peninsula. In these areas organic maturation of the Neogene sediments may have proceeded further than in routes investigated in this study, due to the fact that at over 2, 000 meters thick, the Neogene sediments in these areas are much thicker than in those in this study. Therefore there is a possibility that petroleum has been produced in deep Middle Miocene mudstone in these areas.