PURPOSE: To easily inject a chemical with no need for a battery and electricity by sensing the operating time of a cooling tower by the falling of circulating water, and automatically injecting a constant amt. of the chemical only in the operating time.

CONSTITUTION: When the cooling tower 1 is operated, circulating water passes through a cooling plate 2 and falls into a water reservoir at the lower part. Since a square rod 3 provided with a small bowl 4 is set at the lower part, a larger amt. of water than the water flowing out from a small hole 12 enters the small bowl 4, finally the bowl becomes heavier than the weight 5 of a balance, and the bowl 4 is lowered. A valve 9 closed by the gravity of the weight 5 is opened, and air is taken into a chemical tank 11 through an air intake tube 14, and consequently the chemical is injected into the water reservoir at the lower part of the cooling tower 1 from a chemical inlet tube 10. The state is kept while the cooling tower 1 is operated and circulating water falls.