When a channel formation region is formed of GaN in a MOSFET, there are cases where the actual threshold voltage (Vth) is lower than the setting value thereof and the actual carrier mobility (μ) during the ON state is lower than the setting value thereof. The reason for threshold voltage (Vth) and the carrier mobility (μ) being lower than the setting values is unknown. A MOSFET including a gallium nitride substrate, an epitaxial layer made of gallium nitride provided on top of the gallium nitride substrate, a gate insulating film provided in direct contact with the epitaxial layer, and a gate electrode provided in contact with the gate insulating film. The gallium nitride substrate has a dislocation density less than or equal to 1E+6 cm−2, and the epitaxial layer has a region with a p-type impurity concentration less than or equal to 5E+17 cm−3.