A block copolymer composition comprising intermediate water and a block copolymer, wherein the intermediate water is water in a state in which an exothermic peak based on a low-temperature crystallization of water is observed at or below 0 °C in an elevated temperature process from -100 °C in measurement using a differential scanning calorimeter, the block copolymer comprising an A block and a B block, wherein the A block is capable of containing intermediate water, and the B block is more hydrophobic than the A block, the A block is a polymer block comprising a vinyl monomer-derived structural unit capable of containing intermediate water, in which the vinyl monomer is at least one selected from vinyl monomers represented by the following formulae and: (wherein R 1 is a hydrogen atom, R 2 is alkylene having 2 or 3 carbon atoms, R 3 is methyl or ethyl, and n is 1); (wherein R 7 is a hydrogen atom or methyl); the B block is a polymer block comprising a vinyl monomer-derived structural unit, in which the vinyl monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters, alicyclic alkyl esters of (meth)acrylic acid, (meth)acrylic acid aryl esters, N,N-disubstituted aminoalkyl (meth)acrylate, and aromatic vinyl monomers, and the A block and the B block are present at a molar ratio (A:B) of 85:15 to 40:60. the block copolymer comprising an A block and a B block, wherein the A block is capable of containing intermediate water, and the B block is more hydrophobic than the A block, the A block is a polymer block comprising a vinyl monomer-derived structural unit capable of containing intermediate water, in which the vinyl monomer is at least one selected from vinyl monomers represented by the following formulae and: (wherein R 1 is a hydrogen atom, R 2 is alkylene having 2 or 3 carbon atoms, R 3 is methyl or ethyl, and n is 1); (wherein R 7 is a hydrogen atom or methyl); the B block is a polymer block comprising a vinyl monomer-derived structural unit, in which the vinyl monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters, alicyclic alkyl esters of (meth)acrylic acid, (meth)acrylic acid aryl esters, N,N-disubstituted aminoalkyl (meth)acrylate, and aromatic vinyl monomers, and the A block and the B block are present at a molar ratio (A:B) of 85:15 to 40:60. R 2 is alkylene having 2 or 3 carbon atoms, R 3 is methyl or ethyl, and n is 1); (wherein R 7 is a hydrogen atom or methyl); the B block is a polymer block comprising a vinyl monomer-derived structural unit, in which the vinyl monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters, alicyclic alkyl esters of (meth)acrylic acid, (meth)acrylic acid aryl esters, N,N-disubstituted aminoalkyl (meth)acrylate, and aromatic vinyl monomers, and the A block and the B block are present at a molar ratio (A:B) of 85:15 to 40:60. the B block is a polymer block comprising a vinyl monomer-derived structural unit, in which the vinyl monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters, alicyclic alkyl esters of (meth)acrylic acid, (meth)acrylic acid aryl esters, N,N-disubstituted aminoalkyl (meth)acrylate, and aromatic vinyl monomers, and the A block and the B block are present at a molar ratio (A:B) of 85:15 to 40:60.