To provide a light source apparatus for treating liquid, which has a novel structure and in which a plurality of bar-shaped UV lamps are annularly arranged and a whole periphery in a circumferential direction can be uniformly irradiated with sufficient strength of UV rays.

The light source apparatus for treating liquid is equipped with a sealed outer pipe, a plurality of bar-shaped UV lamps arranged in the inside of the outer pipe and a lamp holding member which holds the UV lamps so as to be annularly arranged along the inner peripheral surface of the outer pipe in an attitude where a lamp central axis extends along the axial direction of the outer pipe. Each UV lamp has a luminous tube in which an aperture part for taking out light outside is formed so as to extend along the lamp central axis at a part in the circumferential direction and is arranged so that its aperture part is directed outward in the radial direction of the outer pipe.