A highly hot-water-resistant antibacterial treatment agent is provided, which is capable of suppressing the sliminess of water supply-and-drainage members such as drainage members, for a long period of time, equipped to a kitchen sink or bathroom drain. The antibacterial treatment agent includes: a quaternary ammonium salt represented by Formula (1) below and a polyhydric carboxylic acid having a C6 or higher hydrocarbon group and two or more carboxyl groups. (In Formula (1), R1 represents a C16-18 alkyl group R2A, R2B, R4A, R4B and R4C each independently represent a C1-3 alkyl group R3 represents a C1-4 alkylene group and X represents a halogen atom).
