A semiconductor device comprises a vertical MOS transistor including a semiconductor substrate having a silicon pillar, a gate electrode formed along a sidewall of the silicon pillar, a gate insulating film formed between the gate electrode and the silicon pillar, an upper diffusion layer formed on the top of the silicon pillar, and a lower diffusion layer formed lower than the upper diffusion layer in the semiconductor substrate; and a pad electrically connected to the lower diffusion layer. Breakdown occurs between the lower diffusion layer and the semiconductor substrate when a surge voltage is applied.