NOVELTY - The method involves humidifying surroundings of a film forming device to absolute humidity of 11.5 gram per cubic meter using a humidifier (30). The forming device is opened to atmosphere. A deposit inside the film forming device is removed. Humidity of the surroundings is measured. The opening of the forming device to the atmosphere is inhibited when the surroundings have the absolute humidity smaller than 11.5 gram per cubic meter. The opening of the forming device to the atmosphere is allowed when the surroundings have the absolute humidity equal to 11.5 gram per cubic meter.    USE - Method for cleaning a film forming device i.e. plasma CVD device, that is utilized to form a silicon-contained thin film on a main surface of a substrate during manufacturing a semiconductor device such as LCD device or thin-film solar cell.    ADVANTAGE - The method enables operating the humidifier to attain the absolute humidity of 11.5 gram per cubic meter or more around the film forming device before an open/close door of a semiconductor manufacturing apparatus opens, so that the air of 11.5 gram per cubic meter in absolute humidity flows into the film forming device when the film forming device opens to the atmosphere, thus enabling the humidified air to increase amount of water on a surface of an object that is transported into the film forming device in a cleaning process, and enabling electric conductivity of the surface of the object to increase to cause rapid leaking of electric charges, for lowering electrostatic potential of the object to suppress generation of static electricity, and hence suppressing oxidation reaction of fine powder of polysilane and the inside film forming to prevent heat generation in a reliable manner for improving the safety during cleaning operation of the film forming device. The method enables an adhering material inside the film forming device to be easily removed from a side wall and a piping by suppressing the static electricity so as to suppress re-adhesion of the adhering material when removed from the side wall and the piping, so that efficiency of the cleaning operation of the film forming device can be improved.    DETAILED DESCRIPTION - The film forming device is a plasma chemical vapor deposition (CVD) device (10). An INDEPENDENT CLAIM is also included for a semiconductor manufacturing apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a semiconductor manufacturing apparatus including a plasma CVD device.    Plasma CVD devices (10, 40, 62-66)    Humidifier (30)    Partition (50)    Transporting device (60)    Semiconductor manufacturing apparatus (100)