An organic electroluminescent device is provided which has a high degree of adhesion between an anode and a hole transport layer, the device emitting light of high luminance even when driven by low applied voltages. Copolymers useful in hole transport layers in organic electroluminescent devices are also provided. Methods of producing hole transport layers for organic electroluminescent devices which can emit light of high luminance even when driven by low applied voltages are also provided, the hole transport layers having high degrees of adhesion with anode. A copolymer comprises a monomer unit including in its structure an electron-donating N,N-diaryl-substituted amino group such as an N,N-diphenylaminophenyl group, or a group having an N,N-diaryl-substituted amino moiety, and a monomer unit having at least one functional group. Also, a hole transport layer, which has excellent thickness accuracy, has an extremely smooth surface, and is very thin, can be formed of this copolymer or a copolymer having the same hole transport functional group. An organic electroluminescent device, which emits light of high luminance even when driven by low applied voltages, can be obtained by using this hole transport layer.