In a method of manufacturing a thin film transistor, when impurity ions are introduced in a channel region between source and drain regions in a semiconductor layer, an insulator layer is first formed on the semiconductor layer. Then, impurity ions generated on high frequency discharge are introduced through the insulator layer into the semiconductor layer under a specified acceleration voltage. Then, the introduction depth of impurities and the amount of the impurities to be introduced in the channel region can be controlled or the threshold voltage of the thin film transistor can be controlled. This method can be applied to a large substrate.