To reduce the size and improve the power added efficiency of an RF power module having an amplifier element composed of a silicon power MOSFET, the on resistance and feedback capacitance, which were conventionally in a trade-off relationship, are reduced simultaneously by forming the structure of an offset drain region existing between a gate electrode and an n+ type drain region of the power MOSFET into a double offset one. More specifically, this is accomplished by adjusting the impurity concentration of an n− type offset drain region, which is closest to the gate electrode, to be relatively low and adjusting the impurity concentration of an n type offset drain region, which is distant from the gate electrode, to be relatively high.