The present invention provides a group of novel SMA derivatives and a covalent conjugate of the derivatives and an active substance. More specifically, the present invention provides: an SMA derivative which contains (i) a styrene-maleic acid copolymer (SMA) and (ii) a side chain (b) that contains a functional group (a) selected from among —NH2, —SH, —OH, —COOH, —NH—(C═NH)—NH2 and —C(CH2—OH)3 and introduced into a carboxyl group of a maleic acid residue of the SMA via an amide bond or an ester bond, and wherein when a plurality of side chain (b) is introduced into the SMA, the side chains (b) may be identical or different from each other and a conjugate of this SMA derivative and an active substance.
