A semiconductor device having a transistor including an oxide semiconductor film is disclosed. In the semiconductor device, the oxide semiconductor film is provided along a trench formed in an insulating layer. The trench includes a lower end corner portion and an upper end corner portion having a curved shape with a curvature radius of longer than or equal to 20 nm and shorter than or equal to 60 nm, and the oxide semiconductor film is provided in contact with a bottom surface, the lower end corner portion, the upper end corner portion, and an inner wall surface of the trench. The oxide semiconductor film includes a crystal having a c-axis substantially perpendicular to a surface at least over the upper end corner portion.