When a start pulse is supplied, a time-constant circuit (1) is charged to a voltage Vcc, and charge in a variable capacitor (C) is discharged through a resistor (R1). Upon the voltage (a) of the variable capacitor (C) reaching a preset reference voltage V1, a signal (b) is output from a comparator (2). In response to the signal (b), a time measuring circuit (3) stores a measured time period as a time period “T1”, and at the same time a switching signal (c) is output. The switching signal (c) switches the reference voltage V1 to a reference voltage V2, which is slightly lower than the reference voltage V1, and further switches the resistor R1 to a resistor R2, which has higher resistance than the resistor R1. Upon the voltage (a) of the variable capacitor (C) reaching the reference voltage V2, the signal (b) is output from the comparator 2. In response to the signal (b), the time measuring circuit (3) stores a measured time period as a time period “T2”.