A protective insulating film in a semiconductor device is formed in a multi-layer structure. A lower layer portion is constituted by an organic-silane-based silicon oxide film formed by a P-CVD process using organic silane and oxygen to improve step coverage. An upper layer portion is constituted by a silane-based silicon oxide film containing excess silicon in an amount greater than that in the stoichiometric composition and formed by a P-CVD process to improve moisture resistance.