A water-soluble composition comprising: an acylphosphinate (A) represented by the following Formula (I): wherein, X 1 represents an aryl group having 6 to 15 carbon atoms; hydrogen atoms in the group represented by X 1 are each optionally substituted with a linear alkyl group having 1 to 8 carbon atoms, a branched alkyl group having 3 to 8 carbon atoms, a linear halogenated alkyl group having 1 to 8 carbon atoms, a branched halogenated alkyl group having 3 to 8 carbon atoms, a linear alkoxy group having 1 to 8 carbon atoms, a branched alkoxy group having 3 to 8 carbon atoms, a linear halogenated alkoxy group having 1 to 8 carbon atoms, or a branched halogenated alkoxy group having 3 to 8 carbon atoms; X 2 is a phenyl group; A m+ is N + HY 1 Y 2 Y 3 ; Y 1 , Y 2 and Y 3 each independently represent a hydrogen atom, a hydroxy group, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 1 to 6 carbon atoms, an aryl group having 6 to 15 carbon atoms, or an arylalkyl group having 7 to 13 carbon atoms; hydrogen atoms in the groups represented by Y 1 , Y 2 and Y 3 are each optionally substituted with a hydroxy group; methylene groups in the groups represented by Y 1 , Y 2 and Y 3 are each optionally substituted with an oxygen atom, a sulfur atom, a carbonyl group, or -N + H-; one or more combinations of Y 1 and Y 2 , Y 1 and Y 3 , and Y 2 and Y 3 are bound with each other to form a ring; and m represents a number of 1 to 3; and a compound (B) having a group represented by the following Formula (II): wherein, R 1 represents a hydrogen atom or a methyl group; Z 1 represents an oxygen atom or -NR 2 -; R 2 represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms; Z 2 represents an alkylene group having 1 to 6 carbon atoms; n represents a number of 0 to 30; * means a bond; and when the compound (B) has plural groups represented by the Formula (II), plural R 1' s, Z 1' s, Z 2' s and n's are each optionally the same or different. an acylphosphinate (A) represented by the following Formula (I): wherein, X 1 represents an aryl group having 6 to 15 carbon atoms; hydrogen atoms in the group represented by X 1 are each optionally substituted with a linear alkyl group having 1 to 8 carbon atoms, a branched alkyl group having 3 to 8 carbon atoms, a linear halogenated alkyl group having 1 to 8 carbon atoms, a branched halogenated alkyl group having 3 to 8 carbon atoms, a linear alkoxy group having 1 to 8 carbon atoms, a branched alkoxy group having 3 to 8 carbon atoms, a linear halogenated alkoxy group having 1 to 8 carbon atoms, or a branched halogenated alkoxy group having 3 to 8 carbon atoms; X 2 is a phenyl group; A m+ is N + HY 1 Y 2 Y 3 ; Y 1 , Y 2 and Y 3 each independently represent a hydrogen atom, a hydroxy group, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 1 to 6 carbon atoms, an aryl group having 6 to 15 carbon atoms, or an arylalkyl group having 7 to 13 carbon atoms; hydrogen atoms in the groups represented by Y 1 , Y 2 and Y 3 are each optionally substituted with a hydroxy group; methylene groups in the groups represented by Y 1 , Y 2 and Y 3 are each optionally substituted with an oxygen atom, a sulfur atom, a carbonyl group, or -N + H-; one or more combinations of Y 1 and Y 2 , Y 1 and Y 3 , and Y 2 and Y 3 are bound with each other to form a ring; and m represents a number of 1 to 3; and a compound (B) having a group represented by the following Formula (II): wherein, R 1 represents a hydrogen atom or a methyl group; Z 1 represents an oxygen atom or -NR 2 -; R 2 represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms; Z 2 represents an alkylene group having 1 to 6 carbon atoms; n represents a number of 0 to 30; * means a bond; and when the compound (B) has plural groups represented by the Formula (II), plural R 1' s, Z 1' s, Z 2' s and n's are each optionally the same or different. <EOS>