PURPOSE: To attain a condition in which the inner wall of a mixer tank is always cleaned, and eliminated the necessity of cleaning work to be done by workers by setting a rotating nozzle for ejecting cleaning water in the interior of the mixer tank and supplying water which is partly utilized as mixing water when a raw material is charged into the tank.

CONSTITUTION: In a mixer tank 1, nozzles 20A, 20B for an automatic cleaning device are provided at two places in spaced relation to each other and are suspended from the upper wall 13 of the mixer tank. A rotating nozzle 23 in the automatic cleaning device 20 has a self-cleaning nozzle header 30 at its upper part connected to a cylinder rod 22 and also a driving unit 31 for nozzles for cleaning the inner wall of the tank at its lower part. The driving unit 31 is provided with a pair of nozzles 32A, 32B for cleaning the inner wall of the tank at the position projected from the side surface of said driving unit, and, furthermore, a self-cleaning nozzle 33 is mounted to the self-cleaning nozzle header 30 in order to clean these nozzles 32A, 32B.