The present invention provides a water photolysis system comprising: a casing 1 into which incident sunlight L can enter from the outside and a photolytic layer 5 which is disposed inside the casing 1; wherein the photolytic layer 5 has a light-transmissive porous material 51 and photocatalyst particles 52 supported thereon; a water layer 4 containing water in its liquid state is disposed below the photolytic layer 5 with a first space 6 disposed between the water layer and the photolytic layer; a sealed second space 7 is formed above the photolytic layer 5 in the casing 1; vapor generated from the water layer 4 is introduced into the photolytic layer 5 via the first space 6; and the vapor is decomposed into hydrogen and oxygen by the photocatalyst particles 52, which are excited by the sunlight L.