The magnetic sensor comprise a multi-layer structure 10 including a ferromagnetic layer 12 of FeCo alloy, an insulation layer 14 of Al.sub.2 O.sub.3 and a compound semiconductor layer 16 of GaAs. Circularly polarized light is irradiated to the compound semiconductor layer 16 to generate electrons. A dc voltage is applied to the ferromagnetic layer 12 and the compound semiconductor layer 16 by a dc power source 20 while circularly polarized light is irradiated to the compound semiconductor layer 16. When a direction of an external magnetic field changes, a magnetization direction of the ferromagnetic layer 12 accordingly changes, and a magnetoresistance between the ferromagnetic layer 12 and the compound semiconductor layer 16 changes. Changes of the magnetoresistance are measured by a voltmeter 22.