The purpose of the present invention is to provide a mesoionic compound, which exhibits an improved controlling effect on various harmful organisms, or a salt thereof. A mesoionic compound represented by formula (1) (wherein: R1 is selected from a hydrogen atom, etc.； R2 represents an optionally substituted phenyl group； R3 is selected from a hydrogen atom, etc.； R4 is selected from a hydrogen atom, etc.； R5 is selected from an optionally substituted ethylene group, etc.； and X is selected from an oxygen atom, etc.) or a salt thereof.
