PURPOSE: To improve heating efficiency by a heating medium by providing a discharge unit generating an ionic wind to mix in the heating medium or a corona electromagnetic field on a heater used for an externally-heated engine.

CONSTITUTION: In a heater 14 used for externally-heated engine like a starting engine, a hydraulic fluid discharged from an engine is heated by the heater 14 by way of a pipe 15 and then returns to the engine by way of a pipe 16. In the heater 14 the hydraulic fluid passes a pipe 14b, and the combustion flame of a fuel oil from a feed pipe 14d flows from a jet 14e to heat a working gas in the pipe 14b. A high voltage introduced into a discharge wire 14c through high-tension generator 20, rectifier 21 and cable 22 applies an ionic wind or a corona magnetic field by corona discharge to a heating medium flowing in the pipe 14b, thus heating the medium rapidly.