PROBLEM TO BE SOLVED: To provide a water purifier that dispenses with a pump for aeration.SOLUTION: A water purifier includes: a light source 1 a light guide plate 2 for receiving the light of the light source 1 at a light incident end 2a an organic substance decomposition photocatalyst ball layer 3 provided on a light emission surface 2b side of the light guide plate 2 and an aeration (oxygen generation) photocatalyst ball layer 4 provided on the organic substance decomposition photocatalyst ball layer 3. Glass plates (or transparent resin layers) 7a and 7b for protecting the light guide plate 2 are provided on both surfaces of the light guide plate 2.