The object of present invention is to provide an immunoassay for dioxins which can rapidly and simply afford measured values having a good correlation with analytical values of dioxins by the official method (GC/MS method). The above object is achieved by using the monoclonal antibody of the present invention having not only a reactivity with the indicator isomer among 17 kinds of PCDDs and PCDFs each having a predetermined WHO-TEF value, but also a high cross-reactivity with several kinds of dioxin isomers having five or six chlorine atoms which contribute largely to a TEQ value, and also having a stable reactivity with the antigens in a measuring solvent.