To provide a pulverizing method of polystyrene foam by which sudden rise of temperature of polystyrene foam at the lower part of a treating tank can be prevented while improving fluidity of polystyrene foam in the treating tank.

By using a pulverizer 32 which is equipped with the tank 2 and a horizontally rotating blade 3 that is provided in the tank 2 and horizontally rotates, the polystyrene foam is charged into the tank 2 and also water is charged into the tank 2 so as to come into contact with the polystyrene foam and then the polystyrene foam containing water is pulverized by rotation of the horizontally rotating blade 3.