To provide an ultraviolet irradiation apparatus that is capable of achieving the downsizing and that is capable of achieving an increase of substantial irradiation time with respect to treatment water by ultraviolet.

The flow channel for treatment water between an inflow hole 1a and an outflow hole 1b of a casing 1 is formed by the gap between an ultraviolet lamp protective tube 3 and a protective tube 4 of an ultraviolet lamp 2, and by the gap between the protective tube 4 and the inner wall of the casing 1. That is, as indicated by an arrow, the treatment water passes through the gap between the ultraviolet lamp protective tube 2 and the protective tube 4 from the inflow hole 1a of the casing 1, then through the gap between the protective tube 4 and the inner wall of the casing 1, and delivered out of the outflow hole 1b of the casing 1.