Two Hall effect devices are formed on a major surface of a silicon single crystal substrate lying in parallel to the (100) crystalline plane and series-connected to form a magnetic sensor. Each of the Hall effect devices has a pair of drive electrodes spaced apart from each other in a direction substantially parallel to the or crystalline axis and held at different potentials for flowing therebetween a drive current in said direction to drive the Hall effect device and a pair of Hall terminals for developing a Hall voltage when exposed to an external magnetic field. A comparator compares the potentials of two selected Hall terminals of the different Hall devices with each other to produce a compared signal. A switching element is connected to one of the Hall devices to control the potential of the Hall terminals to equalize the potentials of the two selected Hall terminals in response to the compared signal. Two non-selected Hall terminals develop positive and negative Hall voltages, respectively, relative to the selected Hall terminals so that the magnetic sensor produces a totalized Hall voltage of the two Hall effect devices.