PURPOSE: To compensate temperature at a place even when the number of Hall elements increases by inserting a resistor having the absolute value of resistance- temperature coefficient larger than the absolute value of the temperature coefficient of Hall element output voltage into a part of the feed-back resistance of an operational amplifier.

CONSTITUTION: An output of the Hall element 4 is once short-circuitted with a constant resistor 9 and then input to the input terminal of the operational amplifier 6 which calculates a sum of the outputs. A semiconductor resistor 10 of silicon semiconductor is inserted into a part of the feed-back resistance of the operational amplifier 6. A combined feed-back resistance value Rf increases in accordance with increase of ambient temperature, causing the amplification factor to increase. Therefore, drop of Hall voltage VH is compensated and a constant current transformer output VO can be obtained for change of ambient temperature. Since a positive temperature coefficient of combined feedback resistance Rf is set to an average negative temperature coefficient of sum of the Hall voltages VH short-circuitted by a resistor 9, the positive temperature coefficient of silicon semiconductor etc. must have an absolute value larger than at least the negative temperature coefficient of the Hall voltage VH.