OVERVOLTAGE PROTECTION DEVICE

An "overvoltage protection device" that protects a device from an overvoltage of a signal line regardless of an ON/OFF state of a power supply is provided. A resistor (R) that is connected in series between an internal signal line (SL) connected to a communication terminal (122) of a processor (12) and a communication line (CL), a diode (D) of which a cathode is connected to the internal signal line (SL) and an anode is connected to a ground (GND), and a PNP transistor (Q) of which a base is connected to a power supply terminal (121), an emitter is connected to the internal signal line (SL), and a collector is connected to the ground (GND) are provided. When a base-emitter voltage (a junction saturation voltage) of the transistor (Q) in operation is defined as V BE and a power source (VDD) is turned on (a voltage V1) by the operation of the transistor (Q), a voltage of the internal signal line (SL) is limited to the source voltage VDD + V BE (a). When the power source (VDD) is turned off (a voltage 0 V), the voltage of the internal signal line (SL) is limited to the source voltage 0 V + V BE (b).