Provided is a novel copolymer that is useful as a pigment dispersant. The novel copolymer comprises block chains (A) containing at least one kind of repeating unit selected from a group consisting of repeating units with tertiary amino groups and repeating units with quaternary ammonium bases, and block chains (B) containing repeating units with acidic groups and repeating units represented by Formula (I) (in the formula, R1 represents a hydrogen atom or C1 to C3 alkyl group and R2 represents an aliphatic hydrocarbon group or alicyclic hydrocarbon group). The proportion copolymerized of the repeating units represented by Formula (I) is 90 mass% or more in the block chains (B).
