To obtain sufficient magnetic field measuring accuracy which is stable for external conditions such as temperature.

The system is constituted of a hall element 1, a first current source 4, a coil 2, a second current source 5 and a control circuit 6. The hall element 1 detects magnetic field intensity and the first current source 4 supplies the hall element 1 with a drive current Ih. The coil 2 is arranged in the vicinity of the hall element 1. The second current source 5 supplies the coil 2 with exiting current Ic. The control circuit 6 controls the drive current Ih and the exiting current Ic. The control circuit 6 controls the current value of the drive current Ih so that the difference between the output value of the hall element 1 when the exiting current Ic is supplied to the coil 2 and the output value of the hall element 1 when the exiting current Ic is not supplied to the coil 2 becomes constant.