A coating agent to prevent nonspecific adsorption of a substance, which comprises a water-soluble copolymer (P) comprising a repeating unit (A) represented by the following formula (1): wherein R 0 represents a hydrogen atom or a methyl group and X represents a group represented by the following formula (1a): wherein R 1 and R 2 independently represent a hydrogen atom or unsubstituted or substituted an alkyl group having from 1 to 8 carbon atoms or formula (1b): wherein R 3 and R 4 independently represent a single bond, substituted or unsubstituted methylene or substituted or unsubstituted divalent alkylene group having from 2 to 7 carbon atoms wherein total of the number of carbon atoms of R 3 and R 4 is from 4 to 10, and Y represents any one of a single bond, O and S a repeating unit (B) represented by the following formula (2): wherein R 5 represents a hydrogen atom or a methyl group and Z represents an organic group containing at least one aldo group or keto group and a hydrazide compound (H) comprising at least two hydrazino groups per one molecule.
