It is an object of the present invention to provide a doping apparatus, a doping method, and a method for fabricating a thin film transistor that can carry out doping to the carrier concentration which is optimum for obtaining the desired electric characteristic non-destructively and in an easy manner. In accordance with the present invention, an electric characteristic of a semiconductor element (threshold voltage in a transistor and the like) is correctly and precisely monitored by using a contact angle, and is controlled by controlling a doping method. In addition, the present invention can be momentarily acquired information by in-situ monitoring the characteristic and can be fed back without a time lag.