A magnetic particle producing part including magnetic and counterpart members in a path of a particle-includable liquid, at least one of the members being moved to press the other member in the liquid, the magnetic member being worn by particles in the liquid to produce magnetic particles; a magnetic particle measuring part positioned in the path same as in the producing part to measure the magnetic particle concentration in the liquid; and a control part converting the concentration of the magnetic particles detected by the measuring part into a concentration of particles in the liquid on the basis of a calibration line representing a correlation preliminarily measured between the concentrations of the magnetic particles and of the particles in the liquid to detect the concentration in the liquid.