To easily turn an apparatus compact and inexpensive and obtain a highly sensitive apparatus by providing a magnetic body and a magnetic sensor, detecting a change of a response waveform corresponding to an intensity of an external magnetic field, and detecting the external magnetic field.

A rectangular wave oscillation voltage Vb is supplied to an integrator circuit from an oscillation voltage feed source 13, whereby an integral current runs to a magnetic sensor 11 and a resistor 12. A Schmitt trigger circuit 14 connected to wirings taken out from both ends of the resistor 12 detects an oscillation voltage Vr generated at the resistor 12 when the integral current runs, and outputs a rectangular wave oscillation voltage Vso as a signal based on a response waveform of the voltage. The voltage Vso output from the circuit 14 is supplied to a logic circuit 15. The voltage Vb from the feed source 13 is also fed to the circuit 15. The circuit 15 compares the voltage Vso from the circuit 14 with the voltage Vb from the feed source 13, and outputs a pulse voltage signal Vp corresponding to a time of the voltage Vr generated at the resistor 12.