Electric characteristics of a semiconductor device using an oxide semiconductor are improved. Further, a highly reliable semiconductor device in which a variation in electric characteristics with time or a variation in electric characteristics due to a gate BT stress test with light irradiation is small is manufactured. A transistor includes a gate electrode, an oxide semiconductor film overlapping with part of the gate electrode with a gate insulating film therebetween, and a pair of electrodes in contact with the oxide semiconductor film. The gate insulating film is an insulating film whose film density is higher than or equal to 2.26 g/cm3 and lower than or equal to 2.63 g/cm3 and whose spin density of a signal with a g value of 2.001 is 2×1015 spins/cm3 or less in electron spin resonance.