A device for utilizing waste heat from an engine, which is provided with a Rankine cycle device having an expander bypass passage and a bypass valve that opens and closes the expander bypass passage, and transmits the output torque Tr of the Rankine cycle device to the engine, estimates the output torque Tr of the Rankine cycle device accurately. An output calculation part includes a torque estimation portion that estimates the torque of an expander. The torque estimation portion has a first torque estimation equation corresponding to an opening position of a bypass valve, and a second torque estimation equation corresponding to a closed position of the bypass valve. The output calculation part calculates an output torque Tr of a Rankine cycle device, based on a torque estimated value by the first or second torque estimation equation.