A semiconductor device includes a power MOSFET including a trench formed on a surface of a semiconductor layer forming a drain; a gate electrode formed in the trench via a gate insulation film and made of poly-silicon; a channel diffusion layer formed at a surface side of the semiconductor layer shallower than the trench by neighboring the trench; and a source diffusion layer formed at a surface side of the channel diffusion layer by neighboring the trench; wherein a reverse impurity layer is provided at a bottom part side of the trench of the poly-silicon forming the gate electrode; and an impurity ion that is a conductive type opposite to the conductive type of an impurity ion provided in the poly-silicon at a surface side of the trench is provided in the reverse impurity layer.