An irradiation apparatus is provided with: a straight pipe that is formed of polytetrafluoroethylene (PTFE) and a light source that is arranged at an end portion of the straight pipe and that irradiates the inside of the straight pipe with ultraviolet light. The light source has a light emitting device that emits ultraviolet light and an adjustment mechanism that adjusts the direction of the ultraviolet light such that the ultraviolet light from the light emitting device becomes incident on the internal wall surface of the straight pipe at an incident angle of 75 degrees or more.
