To execute "rinsing" of instruments with high-temperature "rinse water", and prevent deformation and deterioration of heat-sensitive instruments by sterilizing and eliminating bacteria attached to the instruments by relatively short-time "rinsing" with relatively low-temperature "rinse water" in an instrument washer.

This instrument washer is provided with a preparation device treating feed water and preparing slightly-acid hypochlorous acid water of pH 5-6.5 being usable at a normal temperature and having high sterilizing function; a tank storing the feed water or the prepared slightly-acid hypochlorous acid; and a feed water changeover means changing over the feed water between the preparation device and the tank and feeding the water. This washer executes the "rinsing" of the instruments using the feed water or the slightly-acid hypochlorous acid water.