A copolymer which has both a structural unit derived from a conjugated diene compound and a structural unit derived from an alpha-olefin having 3 to 8 carbon atoms, wherein the melting point of the copolymer falls within the temperature range from 0DEG C to 10 DEG C and the fusion enthalpy of the copolymer is 5 J/g or more as measured with a differential scanning calorimeter under the following conditions: (1) cooling from 200 DEG C to -150 DEG C at a cooling rate of 10oC/min; (2) retaining at -150 DEG C for 1 minute; and (3) heating from -150 DEG C to 200 DEG C at a temperature rising rate of20 DEG C/min.