A fluid heater is configured by passing a cartridge heater H covered by a fluororesin, through a casing 1. The casing 1 has: a fluororesin-made case body 4 which incorporates the cartridge heater H; fluororesin-made lid bodies 5 which are attached to both ends of the case body involving sealing portions S, respectively; fluid supplying/discharging portions 31 which are formed in the lid bodies 5, and through which a fluid is introduced and discharged; and fluororesin-made union nuts 6 which are externally fitted to both ends of the case body 4 and screwed to the lid bodies 5. Sealing portions S formed between the case body 4 and the lid bodies 5 are closely contacted by fastening the union nuts 6 to the lid bodies 5. A lead-out portion 34 for the cartridge heater H is formed in at least one of the paired lid bodies 5.