PURPOSE: To realize the high mobility of a transistor by flattening the interface between a silicon nitride film as a gate insulating film and an amorphous semi conductor film in a thin-film transistor which is used as a driving device or the like for a liquid-crystal flat display.

CONSTITUTION: A film is formed, by a plasma CVD operation, by setting the flow-rate ratio of nitrogen with reference to monosilane at 4 times or lower and by setting the flow-rate ration of hydrogen with reference to monosilane at 13 times or higher, and the unevenness of the surface on the interface side of a silicon nitride film 12 with an amorphous semiconductor film 13 is set at a horizontal cycle of 70nm or higher or the difference between recessed parts and vertical protruding parts is set at a flatness of 3nm or lower. A thin-film transistor manufactured in this manner is provided with a high mobility of 6cm2/V.sec or higher.