A semiconductor device includes a first semiconductor layer formed of a nitride semiconductor on a substrate, a second semiconductor layer formed of a nitride semiconductor on the first semiconductor layer, a gate trench formed in the second semiconductor layer or in the second and first semiconductor layers, a gate electrode formed at the gate trench, and a source electrode and a drain electrode formed on the second semiconductor layer. The gate trench has terminal parts of a bottom of the gate trench formed shallower than a center part of the bottom. A part of a sidewall of the gate trench is formed of a surface including an a-plane. The center part of the bottom is a c-plane. The terminal parts of the bottom form a slope from the c-plane to the a-plane.