To provide a photooxidizer which can efficiently and safely oxidize object components in fluid such as liquid with a simple structure.

An inner tube 1a and an outer tube 1b are sealed at their both ends, and a substance radiating ultraviolet rays by electric discharge is enclosed, thereby forming a ring discharge tube 1. The inside of the inner tube 1a is used as a reaction space 2, into which the liquid is introduced. A reflective membrane 3 is coated around the outer tube 1b. A coil 5 connected to a high-frequency power source 4 is wound around the discharge tube 1. The discharge tube 1 is discharged by an induced magnetic field obtained by supplying a high-frequency current to the coil 5. The liquid is irradiated with the ultraviolet rays radiated from the discharge tube 1 to the inside reaction space 2 to be oxidized.