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, wherein, in the formula, Rl represents a hydrocarbon having 1 to 6 carbon atoms which is substituted with halogen but Rl is not a phenyl group which is substituted with halogen R2 and R7 each independently represent a hydrogen atom, a hydrocarbon having 1 to 6 carbon atoms or an acyl group R3 and R4 each independently represent a hydrogen atom, a hydrocarbon having 1 to 6 carbon atoms which may be substituted or a heteroaryl group which may be substituted, 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 or a hydrocarbon having 1 to 6 carbon atoms 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, wherein said composition is the dust formulation, the emulsifiable concentrate, the wettable powder, the granule or the flowable formulation,and said active ingredients are contained from 0.5 weight % to 20 weight % in said dust formulation, from 0.5 weight % to 50 weight % in said emulsifiable concentrate, from 0.5 weight % to 90 weight % in said wettable powder, from 0.1 weight % to 20 weight % in said granule, and from 0.5 weight % to 90 weight % in said flowable formulation.
