PURPOSE: To manufacture a low-price, high-performance, and smallsized insulating amplifier by amplifying the output of a magnetic detecting means and exciting a 2nd coil, and obtaining an output signal from an amplifying means which eliminates composite magnetic flux.

CONSTITUTION: A Signal applied between input terminals 1 and 3 is amplified by an operational amplifier IC1 and a current is supplied to a primary coil n1 of a transformer T1 through a resistance R4 to produce magnetic flux. This magnetic flux is detected by a Hall element H, which outputs a signal corresponding to the direction and intensity of the magnetic flux; and this signal is amplified by the operational amplifier IC2 and supplied to the secondary coil n2 of the transformer T1 through a resistance R8. Here, a circuit is balanced while the magnetic flux of the transformer T1 is zero unless the sensitivity of the Hall element H is sufficiently high and the gain of the operational amplifier IC2 is sufficiently large. Consequently, an input signal and an output signal are proportional to each other and an input side and an output side are connected in terms of DC, so this amplifier functions as an insulating amplifier.