To provide a dichloroisocyanuric acid tablet releasing quickly high-concentration active chlorine immediately after coming into contact with water, and continuously releasing low-concentration active chlorine over a long period of time after ending the release of the high-concentration active chlorine.

The dichloroisocyanuric acid tablet is to be used, which is such that an alkali metal salt of dichloroisocyanuric acid and sodium metaphosphate are compounded together. In the dichloroisocyanuric acid tablet, the compounding proportion of the alkali metal salt of dichloroisocyanuric acid is 30-80 wt.%, while that of the sodium metaphosphate 1-30 wt.%.