A first silicon oxide film is formed on the surface of the semiconductor substrate in an area of a vertical transfer channel and a read gate contiguous with each other, and a silicon nitride film is formed on the first silicon oxide film. The silicon nitride film is isotropically etched by using a resist pattern formed on the silicon nitride film as a mask. A second silicon oxide film is formed on the surface of the etched silicon nitride film to form an insulating film containing silicon oxide films and a silicon nitride film. A photoelectric conversion element contiguous with the read gate on the opposite side of the vertical transfer channel is formed. The isotropical etching makes the silicon nitride film cover the vertical transfer channel, extend over the read gate, and have a tapered sidewall. A high quality solid state image pickup device can be manufactured.