To stabilize a discrimination process of a signal input for setting an operation mode.

In the process of setting an on threshold for determining whether or not a signal from an input signal line for setting an operation mode is in an on state, a control circuit 12 detects a supply voltage Vs and calculates from the value of Vs a voltage Vopen to be output from a signal processing circuit 10 when the signal input line is open. A voltage Vout₀ to be output from the signal processing circuit 10 when a minimum value of an input voltage Vin indicating a high level is input is also calculated from a potential difference permitted between the input voltage Vin and the supply voltage Vs when the input signal line is connected to a high level power line. A value capable of ensuring a margin to both Vopen and Vout₀ is set as the on threshold.