PURPOSE: To enable the efficient washing of a grinder by supplying water to a water vessel communicated with a powder vessel, and driving a grinding cutter while causing a gradual flow of the water in the case of cleaning the grinding space and the cutter.

CONSTITUTION: When the grinding of coffee beans is just completed, much coffee powder remains adhering to the inner wall surface of a grinding space 1 and each of the cutting edges 4' and 6' of a cutter 7. In this case, a water vessel 11 is filled with water and set to a grinder body 2. Then, the water contained in the vessel 11 is introduced into the grinding space 1 via a communication pipe 12. When a motor 3 is supplied with power in that state, a cutter 7 begins to rotate and the water introduced into the grinding space 1 is splashed with a cutter blade 7-4 and given high velocity energy, thereby colliding with the inner wall surface of the grinding space 1 and a cutter blade 7-6. As a result, the coffee powder remaining as deposited is sufficiently washed away and discharged through a discharge port 9, thereby enabling well washing in a short time.