In a semiconductor device provided with a MOSFET part and a gate pad part defined on a semiconductor substrate which is formed by laminating a low resistance semiconductor layer and a drift layer, the gate pad part includes: the low resistance semiconductor layer; the drift layer formed on the low resistance semiconductor layer; a poly-silicon layer constituting a conductor layer and a gate pad electrode formed above the drift layer over the whole area of the gate pad part with a field insulation layer interposed therebetween; and a gate oscillation suppressing structure where a p-type diffusion region electrically connected with the a source electrode layer and a p-type impurity non-diffusion region are alternately formed on a surface of the drift layer.