Manufactured is a transistor including an oxide semiconductor layer, a source electrode layer and a drain electrode layer overlapping with part of the oxide semiconductor layer, a gate insulating layer overlapping with the oxide semiconductor layer, the source electrode layer, and the drain electrode layer, and a gate electrode overlapping with part of the oxide semiconductor layer with the gate insulating layer provided therebetween, wherein, after the oxide semiconductor layer which is to be a channel formation region is irradiated with light and the light irradiation is stopped, a relaxation time of carriers in photoresponse characteristics of the oxide semiconductor layer has at least two kinds of modes: τ1 and τ2, τ1