A manufacturing method of a semiconductor device includes: forming a tunnel oxide layer and a charge-storage layer in a region of a flash memory transistor; forming a first oxide film; removing the first oxide film in regions of a first transistor and a second transistor; forming a third oxide film by adding a first oxide layer between a first oxide film and a semiconductor substrate in a region of a third transistor while forming a second oxide film in the regions of the first transistor and the second transistor by oxidation; removing the second oxide film in the region of the first transistor; and forming a fifth oxide film by adding a second oxide layer between the second oxide film and the semiconductor substrate in the region of the second transistor while forming a fourth oxide film in the region of the first transistor by oxidation, and forming a sixth oxide film by adding a third oxide layer between the first oxide layer and the semiconductor substrate in the region of the third transistor.