A protection circuit according to the present invention includes: a diode (D1) having an anode thereof connected to a gate signal input terminal and a cathode thereof connected to the gate of an output transistor (N1); a resistor (R1) having one end thereof connected to the gate signal input terminal and the other end thereof connected to ground; a PNP bipolar transistor (Qp1) having the emitter, base and collector thereof connected to the gate of the output transistor (N1), the one end of the resistor (R1) and ground, respectively. With this configuration, it is possible to prevent, without the need for electric power, an open-drain output transistor from erroneously turning on as a result of an electrostatic pulse or the like being applied thereto, and thus to protect the output transistor from electrostatic breakdown.