A manufacturing method of a semiconductor device includes the steps of carrying a substrate in a processing chamber, bringing the processing chamber into a state at a first pressure by supplying a silicon compound gas which contains carbon and hydrogen into the processing chamber, forming a silicon oxide film on the substrate by irradiating a UV light to the silicon compound gas supplied into the processing chamber in the state kept at the first pressure, and decompression process to bring the processing chamber into a state at a second pressure lower than the first pressure. This makes it possible to form the dense silicon oxide film in the trench with high aspect ratio and small width.