An object of the present invention is to provide a composition for preventing harmful organisms exerting an excellent control effect against plant diseases and/or insecticidal effect. Disclosed is a composition for preventing harmful organisms comprising a diamine derivative represented by the formula (1), and one or more compounds selected from the group consisting of other fungicides, insecticides and acaricides as active ingredients, ##STR00001## wherein, in the formula, R1 represents a hydrocarbon having 1 to 6 carbon atoms which is substituted with halogen or the like; R2 and R7 each independently represent a hydrogen atom, a hydrocarbon having 1 to 6 carbon atoms or the like; R3 and R4 each independently represent a hydrogen atom, a hydrocarbon having 1 to 6 carbon atoms which may be substituted or the like, or R3 and R4 represent a cycloalkyl group having 3 to 6 carbon atoms containing a carbon atom bonded thereto; R5 and R6 each independently represent a hydrogen atom, a hydrocarbon group having 1 to 6 carbon atoms or the like; and R8 represents an arylalkyl group which may be substituted, an aryl group which may be substituted or a heteroaryl group which may be substituted.