A first resist mask and a second resist mask used for forming a gate electrode for a p-channel TFT and a gate electrode for an n-channel TFT are left, and a third resist mask is formed afterwards over a first area where one of the p-channel TFT and the n-channel TFT is to be formed; thus, a source region and a drain region are formed in a semiconductor film of the other one of the p-channel TFT and the n-channel TFT by adding first impurity ions using the second resist mask and the third resist mask. After that, the first resist mask, the second resist mask, and the third resist mask are removed, and a source region and a drain region are formed in a semiconductor film of the one of the p-channel TFT and the n-channel TFT by adding second impurity ions using a fourth resist mask.