A fluid treatment device includes a pipe including an inlet, an outlet, an internal space through which a fluid moves and including a light source part disposed in the internal space and providing a light to the fluid. The light source part includes at least one light source unit having a substrate and a plurality of light sources disposed on the substrate and emitting the light. A ratio of a first distance between two light sources adjacent to each other to a second distance between each light source and an inner circumferential surface of the pipe is 1:1.25 or less when viewed in a longitudinal-section.