Provided is a water tank replacement method effective for living organisms that require a water tank with a wall surface and bottom surface that are kept clean. Two water tanks are connected by a linking pipes passing through side walls, larvae are moved to one of the water tanks safely by supplying water to one water tank and discharging water from the other water tank, and inner wall surfaces of the water tank from which the larvae were moved are cleaned after the larvae have gone out. This process is carried out alternately for the two water tanks.
