In the present invention, nickel is selectively extracted from an acidic solution that contains a high concentration of manganese. This valuable metal extraction agent is represented by the general formula. In the formula, R1 and R2 are alkyl groups that may be the same or different, R3 is a hydrogen atom or an alkyl group, and R4 is a hydrogen atom or any group, other than an amino group, bonded to an α carbon atom of an amino acid. The general formula preferably has a glycine unit, a histidine unit, a lysine unit, an aspartic acid unit or a n-methylglycine unit. When extracting nickel by using this extraction agent, it is preferable to adjust the pH of the acidic solution to 2.3 to 5.5 inclusive.