Even when a semiconductor device having field effect transistors driven by relatively different power supply voltages provided over a semiconductor substrate is manufactured by the gate-last process, the breakdown voltage of the transistor on the higher voltage side can be ensured.When forming, over the substrate by the gate-last process, a MOSFET of a core region driven by a first power supply voltage and a MOSFET of a high-voltage region driven by a second power supply voltage higher than the first power supply voltage, the thickness of the hard mask film formed over a dummy gate film of the high-voltage region is made thicker than that of the hard mask film formed over a dummy gate film of the core region, prior to a process of patterning a dummy gate of the MOSFET of the core region and the MOSFET of the high-voltage region. Thereby, the breakdown voltage of MOSFET of the high-voltage region can be ensured.