A semiconductor device with an ESD protection function has an SOI substrate, first to fourth diffusion layers, and a gate. The SOI substrate has a semiconductor layer on an insulation layer. The first diffusion layer is of a first conductivity type and is formed on the semiconductor layer. The second diffusion layer is of the first conductivity type and is formed on the semiconductor layer. The third diffusion layer is of a second conductivity type and is formed on the semiconductor layer so as to be adjacent to the first and second diffusion layers. The fourth diffusion layer is of the second conductivity type and is formed on the semiconductor layer so as to be adjacent to the first diffusion layer and electrically connected to the second diffusion layer. The gate is formed over the third diffusion layer.