PURPOSE: To prevent pollution based on scale without enhancing the concentration of ozone by irradiating a cyclone separator used to remove solid material introduced into a heat exchanger with ultraviolet rays.

CONSTITUTION: Fluid is supplied from the inlet 23 of a cyclone separator 2 and introduced into a body 21 in the tangential direction. A vortex is formed and falls down along a meshy strainer 22 while being rotated. The fluid is stagnated in the lower part of the body 21 and taken out from an outlet 24. Solid material is discharged from an outlet 25 of the lower part. Since rotary flow is caused in the inlet 23 side of fluid, flow velocity is made speedy and scale is made difficult to be stuck. However, flow velocity is made slow in the rear side of the meshy strainer 22 and the outlet 24 side of fluid and scale is easily stuck. Therefore, when the rear side of the meshy strainer 22 and the outlet 24 side of fluid are irradiated with an ultraviolet lamp 26, pollution based on scale is prevented.