When a positive bias voltage is applied to a gate electrode of a transistor including an oxide semiconductor for longer than or equal to 10 msec, electric characteristics of the transistor, which have varied due to the light irradiation, can be brought to the state which is substantially the same as the state before the light irradiation. Note that a positive bias voltage is applied to the gate electrode of the transistor at an appropriate timing with reference to the amount of incident light received by the transistor. Accordingly, a display device in which a reduction in display quality is suppressed even when light irradiation is performed can be realized.