To eliminate a change of an output voltage caused by a change of a capacity of a capacitor, by discharging charges of the capacitor proportionally to an output voltage of a hold circuit and a reference pulse width, when the capacitor is charged in proportion to a measurement pulse width, and a charging voltage is held by the hold circuit to obtain the output voltage after convertion.

A charging circuit charges a capacitor C1 with a constant current Vref/R1 when a switch SW1 is turned on. The switch SW1 is kept on for a measurement pulse width (t). At a hold circuit, a switch SW3 is turned on, an output voltage V1 of an operational amplifier AMP1 at the time is held and an output voltage V2 of an operational amplifier AMP2 is obtained. At a discharging circuit, the switch SW2 is turned on for a period of a reference pulse width T generated from a reference pulse width generation circuit 1, and the capacitor C1 is discharged with a current V2/R2. In other words, charges are discharged in proportion to the output voltage V2 and reference pulse width T. A reduced part of a charging voltage becomes (V2/(R2.C1)) .T, so that a change of the voltage V2 is restricted.