PURPOSE: To obtain a metal surface treatment agent which can firmly adhere to a metal surface and can exhibit a rust-preventive action and an action of bonding to resins even in the form of a thin film by using a polymer obtained by polymerizing a compound having three carbonyl groups and at least one double bond as the effective component.

CONSTITUTION: This metal surface treatment agent is obtained by using a polymer comprising repeating units of formula I (wherein R1 and R2 are each a 1-10C alkyl; R3 is a bond or a 1-8C alkylene; and (x), (y) and (z) are each 0 or 1) or a polymer obtained by polymerizing a compound having three carbonyl groups and at least one double bond and represented by formula II (wherein at least any one of R1, R2 and R4 is a 2-10C double-bond-terminated alkenyl or a 1-10C alkyl; and (x), (y) and (z) are 0 or 1) as the effective component. The keto tricarbonyl groups in the formulae should also include enol ones of the corresponding tautomer. The compound of formula II can be synthesized from a dicarbonyl compound such as 2,4-pentanedione or methyl acetoacetate, acetyl chloride, etc. This polymer is used in the form of an alcoholic solution and is useful for particularly copper foils of printed boards, steel sheets and aluminum sheets.