The present invention is an efficacy-enhancing agent composition for agrochemicals, including water and the following components A, B and C: one or more cellulose derivatives selected from hydroxypropylcellulose and hydroxypropylmethylcellulose, one or more compounds selected from a sulfate, a phosphate, a nitrate, sucrose, glucose, fructose, sorbitol, and glycerin, and one or more surfactants selected from an alkylbenzene sulfonate having an alkyl group with 12 or more and 16 or less carbon atoms and an alkyldimethylamine oxide having an alkyl group with 12 or more and 16 or less carbon atoms,wherein a mass ratio of component B to component C, component B/component C, is 5 or more and 100 or less.
