An antimicrobial agent comprising allyl isothiocyanate (AIT) packaged in a packaging material having a structure wherein part of the pores of a porous packaging substrate is filled with, or said pores are partially or entirely narrowed by a resin impervious to AIT vapor, and a method for controlling the AIT vapor release speed comprising enclosing AIT in the above-mentioned packaging material. According to the present invention, the AIT vapor release speed can be controlled, whereby enabling sustained release of AIT vapor and persistent effect of antimicrobial action. In addition, the antimicrobial agent of the present invention can be widely applied to food industries and various other fields where breeding and reproduction of deleterious microorganisms pose problems, since it is economical, compact and easy to use. The method of the present invention can control the release of the AIT vapor to the outside of the packaging material, to a desired release speed, whereby the antimicrobial action is effectively exerted while preventing the pollution of the environment and damages to the objects to be treated with the antimicrobial agent, which are caused by AIT vapor. In addition, the antimicrobial action can be sustained for a long time with a small amount of AIT.