A method for modifying an insulating film is provided. The method includes a first step of preparing an insulating film containing hydrogen, and a second step of performing microwave treatment on the insulating film to release the hydrogen in the insulating film as water molecules, so that a hydrogen concentration in the insulating film is reduced. Note that the microwave treatment is preferably performed using an oxygen gas and an argon gas at a temperature range of higher than or equal to 200° C. and lower than or equal to 300° C., and the proportion of the flow rate of the oxygen gas to the total of the flow rate of the oxygen gas and the flow rate of the argon gas is preferably greater than 0 % and less than or equal to 50 %.