Abstract: Ice grains of uniform grain size are produced through the change of state of supercooled water and the ice grains are mixed into cold water for obtaining an ice slurry of proper concentration. The ice slurry is injected from an injection means to the surface of a workpiece. As the ice grains are fine and uniform and they are injected with water in the state of ice slurry of stable concentration, a surface processing of a soft workpiece susceptible to damage can be performed without causing damage, and the cleaning of the processed surface is performed concurrently with the surface processing, eliminating the necessity of after cleaning. The apparatus is compact and the initial cost is low. The energy needed for preparing the ice slurry is small.