A method of a semiconductor device, which includes an insulated-gate FET and an electronic element, includes three steps. The first step is the step of forming a trench gate of the insulated-gate FET in a first region of a semiconductor base and a trench element-isolation layer in a second region of the semiconductor base, simultaneously. The second step is the step of forming a first diffusion layer of the insulated-gate FET on a side of the trench gate and a second diffusion layer of the electronic element in a region surrounded by the trench element-isolation layer, simultaneously. The third step is the step of forming a third diffusion layer of the insulated-gate FET in the first diffusion layer and a fourth diffusion layer of the electronic element in the second diffusion layer, simultaneously.