PURPOSE: To precisely measure the mass of iron particle contained in a circulating liquid system by using a coil wound around a magnet as a probe, and measuring the change of inductance value of the coil by the access of iron particle.

CONSTITUTION: A magnet 67 is contained in the body 65 of a sensor 78, and the line of magnetic flux is sent to a collecting surface 60 through a magnetic pole piece 61 to form an attractive magnetic band near the surface 60. When iron particle enters the magnetic band, and it is caught by the surface 60, the inductance value of a coil 62 formed concentrically to the magnetic pole piece 16 is changed. The coil 62 is connected to a serial resonance circuit as an inductor probe, and the change of inductance value changes the frequency of an oscillator based on the serial resonance circuit, and a microprocessor measures this change. This forms a measure for the number of iron particle attracted to the inductor probe by the magnet 67.