PURPOSE: To uniformly clean the whole of a shadow mask by newly attaching a rocking means which rocks a shadow mask inside a wash tank.

CONSTITUTION: When a mask hanger is pitch-fed to dip a shadow mask 1 into the washing water 4 to be stopped in the position opposing a supersonic vibrator 6, the supervibrator 6 is oscillated and a motor 14 is driven, an XY table 13 advances in the direction X so as to insert a hanger instruction part 2a inside a hook 16. In this state, the motors 14, 15 are positively and reversely rotated at a fixed period. when the motor 14 performs positive and reverse rotation, the hook, 16 performs the advance and retreat movement in the X direction to move a hanger supporting part 2a so that the shadow mask 1 is made to rock in the X direction having a part of a rail 3 as a supporting point. When a motor 15 performs positive and reverse rotation, the hook 16 performs the advance and retreat movement in the Y direction to move the hanger supporting part 2a, the shadow mask 1 is made to rock in the Y direction having a pin 2c as a supporting point.