Abstract: The present invention provides a novel copolymer available for a drug delivery technique. More particularly, a novel copolymer for a drug delivery carrier targeting a tumor is provided. The present invention relates to a copolymer comprising structural units represented by the following formulas (A), (B), and (C): wherein R1, R2, and R3 are the same or different and represent a hydrogen atom or a C1-3 alkyl group, R4 represents a C1-3 alkyl group, R5 represents a hydrogen atom, a C1-18 alkyl group, a 3- to 8-membered cycloalkyl group optionally having a substituent, an adamantyl group, a C6-18 aryl group optionally having a substituent, or a 5- to 10-membered heteroaryl group optionally having a substituent, X1, X2, and X3 are the same or different and represent an oxygen atom, a sulfur atom, or N—R7, R6 represents a hydrogen atom, a leaving group, or a linker, R7 represents a hydrogen atom or a C1-3 alkyl group, m represents an integer of 1 to 100, and n represents an integer of 0 to 3.