PURPOSE: To enable efficient washing of a grinding space of a grinder and a cutter thereof by driving a motor while an appropriate amount of water being contained in the grinding space, after closing the discharge port of the grinding space in a water-tight condition.

CONSTITUTION: A freely rotatable seal plate 12 energized with a spring or the like is provided on the tip of a discharge port 11 formed near a cutter 10. The aforesaid seal plate 12 can be manually turned, but when a powder vessel 14 is set, a knob 15 at the upper part of the seal plate 12 is pushed and the plate 12 is automatically released. When the powder vessel 14 is removed after grinding coffee beans or the like, the seal plate 12 turns due to an energizing force from the spring or the like, and closes the tip of the discharge port 11. In this state, when an appropriate amount of water or hot water is supplied to a grinding space 18 and a motor is driven, a cutter 10 begins to rotate and agitates water in the grinding space 18, thereby cleanly washing coffee powder, and coffee powder or the like stagnating at the cutting edge of the cutter 10.