PURPOSE: To improve the resolution and to quicken the conversion speed by comparing an analog input signal with two variable setting values, integrating the difference and converting it into a digital signal depending on the duty ratio with respect to two setting values.

CONSTITUTION: When a setting value Q2 is selected, the value Q2 is converted by a D/A converter 12, a difference (Ex-Q2) with the input signal Ex is integrated by an integration device 13 and its output is compared with a prescribed value by a comparator 14. When an output of the integration device 13 reaches a prescribed value (point Y), the output inverts an output of a terminal Q of an FF15 from "H" to "L" and the said output switches the setting value to the Q1. Then the (Ex-Q1) is integrated and the result is fed to the comparator 14. The output at the terminal Q is at "L" during this period. The FF15 is set again by a set pulse at a terminal S and the setting value Q2 is selected. Then the said operation is repeated and the duty ratio shown in the figure C is outputted from the terminal Q. This operation is repeated until a value (Q2-Q1) becomes a minimum unit, and a digital signal corresponding to the signal Ex is calculated by the value of the said setting value and the duty ratio at an arithmetic circuit 16.