PURPOSE: To provide a stable sensor output by alternately switching the polarity of an applied voltage of a Hall element composed in an IC, and alternately switching the output of a differential detecting amplifier synchronously with switching frequency.

CONSTITUTION: The output electrodes 12, 13 of a Hall element 11 are connected to the bases of transistors (Tr) Q1, Q2 which form a differential detecting amplifier 16 of next stage. A driving current is supplied between the driving electrodes 14 and 15 of the element 11, and the polarity is switched by inverting means 20. Inverting means 20 is connected to the collectors of the Tr Q1, Q2 of the amplifier 16. The means 20, 21 are controlled to be switched in synchronization with a control pulse from a switching signal generator 25, and the switch component is removed by a low pass filter 19. Thus, the cause of the irregularity of an operation magnetic flux density due to the input voltage offset of the amplifier 16 can be reduced.