PURPOSE: To obtain a digital signal accurately proportional to an input analog signal without receiving the effect of a circuit element used for the insulation of a signal simultaneously with a minute analog signal is insulated.

CONSTITUTION: When a pulse signal i0 is applied from a pulse generator OS, currents i1 and i2 flow to a diode circuit D and an input circuit at the side of input terminals 11 and 12 during the rise and fall of the pulse. At the same time, a current iL flows to an inductance. When the current iL which rises or falls linearly flows to the inductance, the signals (e) of pulse widths t1 and t2 can be obtained between output terminals 21 and 22. An arithmetic means CK having a function to obtain the widths t1 and t2 of the pulse signals generated at the terminals 21 and 22 and an arithmetic function CK is connected to said terminals 21 and 22. Thus the widths t1 and t2 are obtained, and at the same time a prescribed calculation is carried out. Thus a digital signal corresponding to an input analog signal Ei is obtained within the digital arithmetic means CK.