A novel triketone derivative having specific formulas [I-1] and [II-1] where in formula [I-1], R represents a methyl group, X and Y are each a hydrogen atom, a halogen atom or the like, M represents a halogen atom, an alkali metal or the like, R4 represents a halogen atom or an alkyl group having 1 to 6 carbon atoms and m is an integer of 0 to 4 and, in formula [II-1], R represents a methyl group, X is a hydrogen atom, a halogen atom or the like, G represents 3 to 5 ring-constituting atoms which form, together with two carbon atoms of an adjacent benzene ring, a 5 to 7-membered saturated or unsaturated condensed ring, Z1 represents a halogen atom or the like, m is an integer of 0 to 4 and q is 1 or 2; and a herbicide containing the same as an effective component. The herbicide has great utility as an efficient herbicide against weeds inhibiting the growth or development of a plant being cultured, particularly lowland weeds such as E. ulilis (Japanese Millet) and S. juncoides.