A copolymer represented by the following formula (I): wherein, A represents a non-substituted or substituted C 1 -C 12 alkoxy group, the substituent in the case of being substituted represents a formyl group or R'R"CH- group, where R' and R" independently represent C 1 -C 4 alkoxy or R' and R" may combine together to form -OCH 2 CH 2 O-, -O(CH 2 ) 3 O- or -O(CH 2 ) 4 O-; L 1 may be selected from groups represented by the formula or may be selected from the group consisting of a single bond, -(CH2)bS-, -CO(CH 2 ) b S-, -(CH 2 ) b NH-, -(CH 2 ) b CO-, -CO-, -OCOO-, and -CONH-; X individually contains a group described in the following (a), (b) and (c): (a) group represented by L 2 -R 1 , wherein L 2 represents -(CH 2 ) a -NH-(CH 2 ) a - or -(CH 2 ) a -O-(CH 2 ) a - and R 1 is represented by any of the following formulas: (b) group represented by L 3 -R 2 , wherein L 3 is the same as defined for L 2 , R 2 is represented by the formula: -(CH 2 ) k -Si(O-Alk) 3 , and each Alk is a C 1-4 alkyl that may be the same as or different from another Alk; (c) group represented by R 3 , wherein R 3 is chloro, bromo, or hydroxy; and units present in a polymer main chain having (a), (b) and (c) are randomly present, units having (a) are within the range of 2 to 99, units having (b) are within the range of 1 to 98, and units having (c) are either not present or within the range of 1 to 20, provided that the total number of these units is n; Z is H, SH, or S(C=S)-Ph, wherein Ph represents phenyl optionally substituted with one or two groups of methyl or methoxy; each a independently represents 0 or an integer of 1 to 5; b represents an integer of 1 to 5; k represents an integer of 1 to 18; m represents an integer of 2 to 10,000; and n represents an integer of 3 to 100. A represents a non-substituted or substituted C 1 -C 12 alkoxy group, the substituent in the case of being substituted represents a formyl group or R'R"CH- group, where R' and R" independently represent C 1 -C 4 alkoxy or R' and R" may combine together to form -OCH 2 CH 2 O-, -O(CH 2 ) 3 O- or -O(CH 2 ) 4 O-; L 1 may be selected from groups represented by the formula or may be selected from the group consisting of a single bond, -(CH2)bS-, -CO(CH 2 ) b S-, -(CH 2 ) b NH-, -(CH 2 ) b CO-, -CO-, -OCOO-, and -CONH-; X individually contains a group described in the following (a), (b) and (c): (a) group represented by L 2 -R 1 , wherein L 2 represents -(CH 2 ) a -NH-(CH 2 ) a - or -(CH 2 ) a -O-(CH 2 ) a - and R 1 is represented by any of the following formulas: (b) group represented by L 3 -R 2 , wherein L 3 is the same as defined for L 2 , R 2 is represented by the formula: -(CH 2 ) k -Si(O-Alk) 3 , and each Alk is a C 1-4 alkyl that may be the same as or different from another Alk; (c) group represented by R 3 , wherein R 3 is chloro, bromo, or hydroxy; and (a) group represented by L 2 -R 1 , wherein L 2 represents -(CH 2 ) a -NH-(CH 2 ) a - or -(CH 2 ) a -O-(CH 2 ) a - and R 1 is represented by any of the following formulas: (b) group represented by L 3 -R 2 , wherein L 3 is the same as defined for L 2 , R 2 is represented by the formula: -(CH 2 ) k -Si(O-Alk) 3 , and each Alk is a C 1-4 alkyl that may be the same as or different from another Alk; (c) group represented by R 3 , wherein R 3 is chloro, bromo, or hydroxy; and units present in a polymer main chain having (a), (b) and (c) are randomly present, units having (a) are within the range of 2 to 99, units having (b) are within the range of 1 to 98, and units having (c) are either not present or within the range of 1 to 20, provided that the total number of these units is n; Z is H, SH, or S(C=S)-Ph, wherein Ph represents phenyl optionally substituted with one or two groups of methyl or methoxy; each a independently represents 0 or an integer of 1 to 5; b represents an integer of 1 to 5; k represents an integer of 1 to 18; m represents an integer of 2 to 10,000; and n represents an integer of 3 to 100. <EOS>