An output transistor bias generation circuit which applies a bias voltage to one of two NMOS transistors constituting an output circuit having a stack structure, includes diode-connected NMOS transistors provided between an external connection pad connected to an external signal line having a voltage higher than a power supply voltage of an LSI circuit, and the gate of an NMOS transistor, diode-connected NMOS transistors provided between the gate of the NMOS transistor and a ground line, a diode-connected NMOS transistor provided between the power supply line and the gate of the NMOS transistor, and a capacitor-connected NMOS transistor provided between the gate of the NMOS transistor and the ground line.