To allow a chemical to spread over the whole cooling system to uniformize the concentration of the chemical and to effectively control slime, and to prevent corrosion of a metallic material by injecting a halogen type chemical into at least two places of the vicinity of a water discharge part to a circulation pump in a cooling water tank and of the place except the vicinity of the water discharge part.

The place being injected the halogen chemical is at least two places where are the vicinity of the water discharge part (a position b or an overflow part 7) to the circulation pump in the cooling water tank and the places (positions a, c-f) except the vicinity of the water discharge part. At this time, as the place except the vicinity of the water discharge part, the places (positions d, e, f) at the opposite side to a water discharge part side of the cooling water tank, are preferred, and besides the confronted place (the position e) to the vicinity of the water discharge part is further preferred. A residual chlorine concentration at the vicinity of the water discharge part (the position b or the over flow part 7) is preferred to be 0.1-1.0 mg/liter in respect to the circulated water quantity of the circulation pump.