To obtain a new antimicrobial agent having remarkably better antimicrobial actions on bacteria, molds, yeasts, etc., than those of naturally occurring catechins and a new catechin derivative and to provide a method for producing the catechin derivative.

This antimicrobial agent comprises the catechin derivative represented by the formula (I), R1 and R2 denote each independently hydrogen atom or CH(SCH3)R7, with the proviso that both are not hydrogen atom; R3 denotes hydrogen atom, an alkyl group which may be substituted or an acyl group; R4, R5 and R6 denote each independently hydrogen atom, hydroxy group, an alkoxy group or an acyloxy group; R7 denotes an alkyl group which may be substituted; and Ra and Rb denote each independently hydrogen atom, an alkyl group which may be substituted or an acyl group].