Provided is a transistor which includes an oxide semiconductor film in a channel region. A change from a shift value before light irradiation to a shift value under light irradiation is greater than or equal to −1 V and less than or equal to 0.5 V, where the shift value is a gate voltage at a point of intersection of an axis of 1×10−12 A and a steepest tangent line of the logarithm of a drain current in drain current-gate voltage characteristics of the transistor, and where the light irradiation is performed on the oxide semiconductor film with light having an energy greater than or equal to a band gap of the oxide semiconductor film.