PURPOSE: To convert the input analog quantity into a digital quantity with high stability, by digitally charging a charge storage device and comparing the charged voltage with an input voltage.

CONSTITUTION: A constant current circuit 5 feeds a given current to a charge storage element 3 every time the period pulse from a pulse generating circuit 6 inputs. Thus, the terminal voltage at the element 3 increases stepwise. This voltage is given to one input terminal of a comparator 4 and an input analog voltage is given to the other input terminal. When both the inputs to the comparator 4 are equal, a pulse output (b) is produced and the gate circuit 7 is closed, allowing to count the number of pulses produced at a pulse generating circuit 6 until the time point at the count circuit 8.