PURPOSE: To perform heating, by exchanging heat between the pressurized and heated oil by a hydraulic pump and the fluid to be heated.

CONSTITUTION: The oil pressurized and heated by the gear pump 14 is discharged into the oil in the oil tank 22 from the high pressure nozzle 20 through the relief valve 16 and the adjusting valve 18, and returned to the normal air pressure. Through the heat exchanger 38 provided with the oil tank 22, the heated oil is exchanged with the fluid 40-42 to be heated. Then, the cooled oil returned to the gear pump 14. The oil temperature in the oil tank 22 is sensed by the sensor 34, and the motor 12 is controlled by the sensed results, by which the drive of the gear pump is also controlled. Because fuel is not used and the air is not polluted, this device is safely and can be used for the heating apparatus, etc.