The fluid heater

PROBLEM TO BE SOLVED: To provide a fluid heater which is high in heat efficiency, and excellent in reliability.SOLUTION: A fluid heater 1 comprises a flow passage unit 10 having a plurality of parallel flow passages 11a, 11b therein, and suitably-linear state heat generation bodies 20 which are accommodated in a plurality of the flow passages 11a, 11b, respectively, and coated with electrical insulation materials. A plurality of the flow passages 11a, 11b are made to communicate with each other in series. It is preferable that lid parts having communication paths 12 for making the plurality of the flow passages 11a, 11b communicate with each other two by two are arranged at the flow passage unit 10 at least at one-side ends of the plurality of the flow passages 11a, 11b in an extension direction. Furthermore, a flow passage in which the heat generation body 20 is not accommodated may be formed at the flow passage unit 10 in a state that a periphery of the flow passage is surrounded by the plurality of flow passages.