A block copolymer comprising a polyalkylene glycol block and a polyhydroxyalkanoic acid block, wherein the mass ratio of the polyalkylene glycol block with respect to the total mass is 10 to 50%, wherein said mass ratio is determined by Nuclear Magnetic Resonance (NMR) as described in the description; the carbonyl carbon has a carbon nuclear relaxation time T1ρ of not more than 20 ms, wherein carbon nuclear relaxation time T1ρ is determined by Solid-State NMR as described in the description; and the block copolymer satisfies the following Equation 1: X = X 1 × X 2 > 20 χ1: crystallization rate of the polyalkylene glycol block; χ2: crystallization rate of a poly-A block, wherein A represents, among the repeat units contained in the polyhydroxyalkanoic acid block, a repeat unit whose homopolymer composed of the same repeat units has the highest crystallization rate, wherein χ, χ1 and χ2 are determined by Differential Scanning Calorimetry (DSC) as described in the description, wherein the polyhydroxyalkanoic acid block comprises a repeat unit derived from a monomer selected from the group consisting of lactic acid, glycolic acid, and caprolactone, and the block copolymer is represented by the following General Formula (I): wherein A represents a polyhydroxyalkanoic acid block; D represents C or a single bond, C represents a linker molecule containing two or more carboxyl groups, B' represents B or a block of two or more B's linked to each other through C(s) with the proviso that the molecule necessarily contains a block of two or more B's linked to each other through C(s), and B represents a polyalkylene glycol block; and n represents an integer of 1 or more. the mass ratio of the polyalkylene glycol block with respect to the total mass is 10 to 50%, wherein said mass ratio is determined by Nuclear Magnetic Resonance (NMR) as described in the description; the carbonyl carbon has a carbon nuclear relaxation time T1ρ of not more than 20 ms, wherein carbon nuclear relaxation time T1ρ is determined by Solid-State NMR as described in the description; and the block copolymer satisfies the following Equation 1: X = X 1 × X 2 > 20 χ1: crystallization rate of the polyalkylene glycol block; χ2: crystallization rate of a poly-A block, wherein A represents, among the repeat units contained in the polyhydroxyalkanoic acid block, a repeat unit whose homopolymer composed of the same repeat units has the highest crystallization rate, χ1: crystallization rate of the polyalkylene glycol block; χ2: crystallization rate of a poly-A block, wherein A represents, among the repeat units contained in the polyhydroxyalkanoic acid block, a repeat unit whose homopolymer composed of the same repeat units has the highest crystallization rate, wherein χ, χ1 and χ2 are determined by Differential Scanning Calorimetry (DSC) as described in the description, wherein the polyhydroxyalkanoic acid block comprises a repeat unit derived from a monomer selected from the group consisting of lactic acid, glycolic acid, and caprolactone, and the block copolymer is represented by the following General Formula (I): wherein A represents a polyhydroxyalkanoic acid block; D represents C or a single bond, C represents a linker molecule containing two or more carboxyl groups, B' represents B or a block of two or more B's linked to each other through C(s) with the proviso that the molecule necessarily contains a block of two or more B's linked to each other through C(s), and B represents a polyalkylene glycol block; and n represents an integer of 1 or more. <EOS>