An agent for hair deforming treatment comprising the following components, and: glyoxylic acid, or a hydrate or a salt thereof, one or two or more member selected from the compounds represented by the following formula: wherein R 1 represents a hydrogen atom or a methyl group, X represents a hydrogen atom, a hydroxy group, or a methoxy group, Y represents a hydrogen atom, an oxygen atom, a hydroxy group, or a methoxy group, Z represents a hydrogen atom, or a straight-chain or branched-chain alkyl group or alkenyl group containing from 1 to 5 carbon atoms, R x represents a hydrogen atom, an oxygen atom, a hydroxy group, a methoxy group, or an aromatic hydrocarbon group, which is optionally substituted with up to three hydroxy groups or methoxy groups and which optionally forms a condensed ring with 1,3-dioxolane, R y represents a hydrogen atom, a hydroxy group, a methoxy group, or an aromatic hydrocarbon group, which is optionally substituted with up to three hydroxy groups or methoxy groups and which optionally forms a condensed ring with 1,3-dioxolane, or an arylcarbonyloxy group or aralkylcarbonyloxy group, which is optionally substituted with up to three hydroxy groups or methoxy groups, and the dashed line represents that the portion is optionally a double bond, provided that the dashed line and the solid line, which are adjacent to R x or Y, each represent a double bond only when R x or Y represents an oxygen atom, and otherwise these lines each represent a single bond, and provided that Z is a straight-chain or branched-chain alkyl group or alkenyl group containing from 1 to 5 carbon atoms only when R x or R y is an o,p-dihydroxy aromatic hydrocarbon group, and otherwise Z is a hydrogen atom, and water, wherein the molar ratio of the content of the component to the content of the component, /, is 0.01 or more and 5 or less; and wherein the content of the component is 2.5 mass% or more and 30 mass% or less. glyoxylic acid, or a hydrate or a salt thereof, one or two or more member selected from the compounds represented by the following formula: wherein R 1 represents a hydrogen atom or a methyl group, X represents a hydrogen atom, a hydroxy group, or a methoxy group, Y represents a hydrogen atom, an oxygen atom, a hydroxy group, or a methoxy group, Z represents a hydrogen atom, or a straight-chain or branched-chain alkyl group or alkenyl group containing from 1 to 5 carbon atoms, R x represents a hydrogen atom, an oxygen atom, a hydroxy group, a methoxy group, or an aromatic hydrocarbon group, which is optionally substituted with up to three hydroxy groups or methoxy groups and which optionally forms a condensed ring with 1,3-dioxolane, R y represents a hydrogen atom, a hydroxy group, a methoxy group, or an aromatic hydrocarbon group, which is optionally substituted with up to three hydroxy groups or methoxy groups and which optionally forms a condensed ring with 1,3-dioxolane, or an arylcarbonyloxy group or aralkylcarbonyloxy group, which is optionally substituted with up to three hydroxy groups or methoxy groups, and the dashed line represents that the portion is optionally a double bond, provided that the dashed line and the solid line, which are adjacent to R x or Y, each represent a double bond only when R x or Y represents an oxygen atom, and otherwise these lines each represent a single bond, and provided that Z is a straight-chain or branched-chain alkyl group or alkenyl group containing from 1 to 5 carbon atoms only when R x or R y is an o,p-dihydroxy aromatic hydrocarbon group, and otherwise Z is a hydrogen atom, and R 1 represents a hydrogen atom or a methyl group, X represents a hydrogen atom, a hydroxy group, or a methoxy group, Y represents a hydrogen atom, an oxygen atom, a hydroxy group, or a methoxy group, Z represents a hydrogen atom, or a straight-chain or branched-chain alkyl group or alkenyl group containing from 1 to 5 carbon atoms, R x represents a hydrogen atom, an oxygen atom, a hydroxy group, a methoxy group, or an aromatic hydrocarbon group, which is optionally substituted with up to three hydroxy groups or methoxy groups and which optionally forms a condensed ring with 1,3-dioxolane, R y represents a hydrogen atom, a hydroxy group, a methoxy group, or an aromatic hydrocarbon group, which is optionally substituted with up to three hydroxy groups or methoxy groups and which optionally forms a condensed ring with 1,3-dioxolane, or an arylcarbonyloxy group or aralkylcarbonyloxy group, which is optionally substituted with up to three hydroxy groups or methoxy groups, and the dashed line represents that the portion is optionally a double bond, provided that the dashed line and the solid line, which are adjacent to R x or Y, each represent a double bond only when R x or Y represents an oxygen atom, and otherwise these lines each represent a single bond, and provided that Z is a straight-chain or branched-chain alkyl group or alkenyl group containing from 1 to 5 carbon atoms only when R x or R y is an o,p-dihydroxy aromatic hydrocarbon group, and otherwise Z is a hydrogen atom, and water,