An overvoltage protection circuit includes a PMOS transistor Q1 that is disposed between an input circuit 21, from which an input voltage VIN is supplied, and a system 22 and functions as a switch. A comparator 110 compares the input voltage VIN with a predetermined reference voltage to determine the occurrence of an overvoltage. Further, the comparator 110 outputs a High level as an operation signal when no overvoltage is detected. A soft switching control circuit 130 starts up by using the High level output from the comparator 110 as an enable signal, and gradually turns on the PMOS transistor Q1. A sudden change of the load exerted on the circuit on the input side is suppressed by the soft start function.