To provide a solid drug to be put in a water circuit within a handpiece, in the circumference of which water is made to flow and the drug melts in the water causing the aqueous disinfectant mixed with the drug to pour from the handpiece.

A pill 100 is a water-soluble kneaded mixture with a disinfectant, and this mixture (pill) 100 is put in a water circuit 110 within a dental treatment handpiece. During a dental treatment when rinsing water or cooling water is sprayed into a target area for treatment, the disinfectant mixed in the pill 100 is dissolved, resulting in the spray of the rinsing water or cooling water mixed with the disinfectant. This pill 100 may be thrown directly in the water circuit 110, but throwing a water-permeable container 101 (e.g. reticulate) containing the pill 100 in the water circuit will serve to prevent the water circuit from blocking up.