To reduce manufacturing cost of a semiconductor device.

In a vapor-phase growth method used for forming a thin film of a semiconductor, an N-type or P-type impurity is introduced into a thin film of the semiconductor during the film formation, by simultaneously supplying a gas containing an element of the P-type or N-type impurity and a source gas for the semiconductor from a gas cylinder filled with a gas containing the element of the P-type or N-type impurity mixed in the source gas for the semiconductor. Thereby, the amount of N-type or P-type impurity, necessary for making the threshold-voltage of the thin-film transistor a desired value, is introduced into the thin film of the semiconductor during the film formation, without having ion implantation.