A novel material is provided. A composite oxide semiconductor in which a first region and a plurality of second regions are mixed is provided. Note that the first region contains at least indium, an element M (the element M is one or more of Al, Ga, Y, and Sn), and zinc, and the plurality of second regions contain indium and zinc. Since the plurality of second regions have a higher concentration of indium than the first region, the plurality of second regions have a higher conductivity than the first region. An end portion of one of the plurality of second regions overlaps with an end portion of another one of the plurality of second regions. The plurality of second regions are three-dimensionally surrounded with the first region.