Abstract: A fluid heater, capable of having a high energy density structure with an increased radiation receiving area, facilitating maintenance services and preventing intrusion of contamination substances, comprises a transparent internal pipe (11) which is open-ended at both ends and is positioned within an external pipe (12) of a fluid heating pipe unit (1). The fluid heating pipe unit (1) is provided with a fluid inlet (31) and a fluid outlet (32). The adjacent ends of the external pipe (12) and internal pipe (11) are connected to each other and sealed, and an electrical radiant heater (2) is arranged within the internal pipe (11) with a space (7) left therebetween. The ends of the electric heater (2) are removably supported by positioning members (91, 92). An inwardly directed radiation reflecting surface (12b) and/or a radiation absorbing material (12c) can be provided as part of the external tube (12).