PROBLEM TO BE SOLVED: To provide an agent which can deactivate microorganisms or reduce the number of viable cells in a microbial agrochemical at timing desired by a user, is simple, sure and safe, and does not cause a secondary pollution problem. SOLUTION: This agent for reducing the viable cell number of the microbial agrochemical is characterized by containing the water-soluble derivative of chitosan as an active ingredient. The agent is prepared by adding the water-soluble derivative of chitosan to the suspension of the microbial agrochemical. COPYRIGHT: (C)2004,JPO&NCIPI