A physiologically active substance-conjugated block copolymer having enhanced efficacy and/or safety is provided by enhancing the property of penetrating into a target diseased tissue and/or enhancing excretability, compared to known physiologically active substance-conjugated block copolymer, and suppressing sensitization of the physiologically active substance to normal tissues other than a target diseased tissue. Disclosed is a block copolymer including a polyethylene glycol segment connected with a polyamino acid derivative segment conjugated with a physiologically active substance, in which the molecular weight of the block copolymer is from 2 kilodaltons to 15 kilodaltons, and the light scattering intensity of a 1 mg/mL aqueous solution of the physiologically active substance-conjugated block copolymer as measured with a laser light scattering photometer is at least twice or more the light scattering intensity of toluene.
