PURPOSE: To improve the accuracy of a pulse width, by performing the setting of the 2nd reference voltage determining the pulse width of a detected pulse, independently of the setting of the 1st reference voltage determining the mask width.

CONSTITUTION: When an input pulse S1 is incoming, an output S15 of an input circuit 61 falls down to set a switching driving circuit 55. Then, a discharging constant current source 53 is turned on to start discharging a capacitor 51. When a voltage VC across the capacitor 51 reaches a lower limit reference voltage VL or below of a lower limit comparison circuit 58, the switching driving circuit 55 is reset, the discharging constant current source 53 is turned off, a charging constant current source 52 is turned on and the charging of the capacitor 51 is started. When the voltage VC across the capacitor 51 exceeds the upper limit reference voltage VH of an upper limit comparison circuit 56, the comparison circuit 56 is inverted, and an output S13 is risen.