PURPOSE: To prepare a blue pigment-incorporated dry oxidizing agent for water system sterilization by blending a specific quantity of 1,3-dichloro-5,5- dimethylhydantoin, potassium bromide, calcium hypochlorite and lapis-lazuli.

CONSTITUTION: The blue pigment-incorporated dry oxidizing agent is prepared by blending 99.5-99.99wt.% halogen releasing compound A selected from a mixture of 1,3-dichloro-5,5-dimethylhydantoin, trichloro-s-triazinetrione and sodium bromide, a mixture of sodium dichloro-s-triazinetrione and sodium bromide, 1-bromo-3-chloro-5,5-dimethylhydrantoin, calcium hypochlorite and the like and 0.01-0.5wt.% lapis-lazuli stone of the blue pigment. The blended quantity of the A is suitably 99.75-99.5wt.% and the blended quantity of the B is suitably 0.50-0.25wt.%.