The problem to be solved by the present invention is to provide a novel mixed refrigerant that can be an alternative for HCFC-123, has a low ODP and a low GWP, has a COP equal to that of HCFC-123, and has a refrigerating capacity and gas density higher than those of single refrigerants, such as HCFO-1233zd(E) and HFC-245fa, which are being considered as alternative refrigerants for HCFC-123. A composition comprising a refrigerant, wherein the refrigerant comprises HCFO-1233zd(E) and HFC-245fa, and has a mass ratio of HCFO-1233zd(E) to HFC-245fa of 18:82 to 49:51 is provided as a means for solving the above problem.