A method for forming a semiconductor device including a semiconductor layer, formed of a silicon-based deposited film containing crystals by plasma-enhanced CVD, includes the steps of applying a bias voltage between a high-frequency electrode and a substrate with the high-frequency electrode being negative when the semiconductor layer is formed; detecting sparks occurring on the high-frequency electrode or the substrate; and controlling at least one condition, selected from the group consisting of high-frequency power, bias voltage, bias current, pressure, gas flow rate, and interelectrode distance, based on the results of the detection so that the number of sparks with durations of 100 ms or more is 1 or less sparks per minute.