PURPOSE: To prevent the scattering of dust from the upper surface of a housing by forming an air curtain above a processing part.

CONSTITUTION: A housing 1 is constituted of supports 2 and a cross beam 3. A processing part 4 is provided inside the housing 1 and a raw stone W is cut by cutting blades 5 while water containing iron sand is sprinkeld from a water sprinkler 6. The periphery of the housing 1 is closed by lateral leak preventing sheets and closure plates 32. A blower 16 is arranged above the processing part 4 and air blowoff ports 8 are formed to the side part thereof to form an air curtain 30 above the processing part 4. An obique guide plate 26 guiding air streams downwardly is horizontally provided to the part opposed to the blower 6.