To provide an antimicrobial activity reinforcement which reinforces the antimicrobial activity of an organic antimicrobial agent and, even when used with a low concentration over a long period, does not allow microbes to acquire resistance/durability.

The antimicrobial activity reinforcement, used for reinforcing an organic antimicrobial agent, is a quaternary ammonium salt compound represented by formula (1); and an antimicrobial ingredient containing the organic antimicrobial agent and the compound represented by formula (1) is also provided. In formula (1), R1-R6 are each independently an optionally branched 1-20C alkyl group; Y is not present or is a 1-12C alkylene group optionally having an ether bond; Xn- is an inorganic or organic anionic group; and n is the valence number of the anionic group Xn-, the product of m and n being 2.