PURPOSE: To enable an insulating film, a semiconductor film and a semiconductor protective film in high quality to be formed successively by a method wherein a substrate is carried to the first - the third chambers communicating with one another through the intermediary of opening and closing doors as well as a continuous plasma CVD device provided with a cathode electrode sheet, an exhaust pipe and a gas feeder pipe arranged in respective chambers to be filmed.

CONSTITUTION: A glass substrate 11 is successively carried to the first - third chambers 1-3 without exposing to the atmosphere to perform the plasma CVD process in respective chambers 1-3. Thus, an SiO2 film 13, an α-Si film 14 and an Mo film 15 in excellent film quality subjected to no pollution etc., are respectively filmed on the glass substrate 11. In other words, a three layer thin film containing the α-Si film 14 subjected to no pollution can be formed efficiently by simple operations due to a semiconductor region 16 etc., extremely important for the operations of a thin film transistor and vulnerable to pollution in atmosphere as well as the capability in carriage of the α-Si film 14 after formation to the next filming process without exposing to atmosphere. Consequently, such a three layer thin film, after formation can be processed e.g., patterning etc., so that a thin film transistor in stable characteristics and high quality may be formed in high yield.