Provided is a plasma CVD device. In the plasma CVD device, in producing a silicon nitride film while controlling the size of a band gap by CVD, microwaves are introduced into a treatment vessel by a flat antenna having a plurality of holes. The plasma CVD is carried out under a given treatment pressure selected from a pressure range of not less than 0.1 Pa and not more than 1333 Pa at a flow ratio between a silicon-containing compound gas and a nitrogen gas (silicon-containing compound gas flow rate/nitrogen gas flow rate) selected from a range of not less than 0.005 and not more than 0.2, whereby the Si/N ratio in the film is controlled to form a silicon nitride film having a band gap size of not less than 2.5 eV and not more than 7 eV.