PURPOSE: To provide a washing method for a cell pollution material in a flow sight meter wherein a strong oxidation solution, a non-particle neutral pH fluid and a weak acid are sequentially used.

CONSTITUTION: A reservoir 80 contains a strong oxidation solution, and a reservoir 81 contains a neutral pH fluid, and a reservoir 82 contains a weak acid. The air which pressurizes the reservoir is supplied by a pump 83 in a pipe channel provided with a filter 84. A rotation valve 85 is so provided as to direct an air flaw to each reservoir. In addition, with another rotation valve 86, the reservoir is connected to a fluid system.