Apparatus for utilizing heat wasted from an engine includes first pressure detecting means (73) and first temperature detecting means (81) for detecting a pressure and a temperature in a refrigerant passage extending from a condenser (38) to a refrigerant pump (32), second pressure detecting means (72) and second temperature detecting means (82) for detecting a pressure and a temperature in a refrigerant passage extending from a heat exchanger (36) to an expansion device (37), and control means (71) responsive to these four detecting means when operating a Rankine cycle (31). Means (71) is for diagnosing whether or not an electromagnetic clutch (35) is stuck responsive to either the first pressure detecting means (73) and the first temperature detecting means (81), or the second pressure detecting means (72) and the second temperature detecting means (82).