In a first embodiment, Tetraethyl Orthosilicate Si(OC2H5)4 is used at the process temperature of 650 C.5 C. as film forming material, to decrease crystal defects occurring during deposition. In a second embodiment, annealing is carried out in sparse oxygen gas atmosphere after deposition, to mend crystal defects that occurred during deposition. In a third embodiment, initial temperature of the CVD device is kept at about 400 C., whereby the start of natural oxidation of the deposition surface is prevented and production circumstances of the semiconductor element is not deteriorated. Then, the CVD device is heated up to CVD temperature of about 750 C. or about 650 C., to deposit oxide.