The present invention provides a novel SMA derivative and a covalent conjugate of the derivative 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 in cases where a plurality of the side chains (b) are introduced into the SMA, the side chains (b) may be the same as or different from each other； and a conjugate of this SMA derivative and an active substance.
