PURPOSE: To detect a particle in a liquid metal by erasing an interference constituent that is superposed on a test signal for detecting the passage of a particle with different conductivity through a path.

CONSTITUTION: Electrodes 12, 14, and 15 are provided in a sampling head 10, and the electrode 12 is inserted into a sampling tube 16, and the electrodes 14 and 15 are inserted into a melted metal 18. The melted metal 18 flows through a container 20 and is pulled into the tube 16 through a path 22 due to vacuum created by a vacuum source 26 and a DC power supply 28 supplies a DC current to the electrodes 12 and 14. The electrodes 12 and 14 detect a change in a voltage difference after a particle with different conductivity passes through the path 22 and supply a test signal to a differential preamplifier/filter circuit 40, and the electrodes 12 and 15 supply a signal for eliminating an interference signal that is superposed on a test signal to the circuit 40. Then, the circuit 40 adds an interference erasure signal to a test signal and reduces an interference signal, the output of the circuit 40 is supplied to a calculation and recording device 46 via a variable gain amplifier 42 and a LPF 44, and the device 46 can generate a visible recording for indicating the number of particles per unit volume of a metal.