Disclosed herein is a light source device using light-emitting diodes (LEDs). Specifically, a two-sided, surface light source device is implemented with point light sources such as LEDs in a simple configuration, such that even surface light is emitted. The two-sided, surface light source device includes: a substrate on which light-emitting diodes (LEDs) are disposed as a light source; and a two-sided diffusion unit having a plate-like shape with a front side face, a back side face, and a light-receiving face that is perpendicular to the front side face and the back side face and faces the substrate. Light emitted from the light source is diffused in the diffusion unit and exits through the front side face and the back side face.