The present invention addresses the problem of providing a block copolymer conjugate of a physiologically active substance wherein the permeation to and/or excretion from a target disease tissue are improved compared with publicly known block copolymer conjugates of physiologically active substances and sensitization of healthy tissues other than the target disease tissue by the aforesaid physiologically active substance is suppressed so as to improve efficacy and/or safety. The block copolymer comprises a polyethylene glycol segment and a polyamino acid derivative segment to which the physiologically active substance is bound, wherein: the molecular weight is 2-15 kDa inclusive； and the light scattering intensity of a 1 mg/ml aqueous solution of the block copolymer conjugate of the physiologically active substance is twice or more as high as the light scattering intensity of toluene when measured with a laser light-scattering photometer.
