A planar light source includes an irregular-shaped mounting substrate, light sources arranged along first and second directions in a plan view, and a light diffusion plate including a thick plate portion and a thin plate portion outward of outermost light sources in the plan view. In the first direction, a distance between an optical axis of one first distal-end light source of one array and an outer edge of the light diffusion plate is greater than a distance between that of another first distal-end light source of another array and the outer edge. In a plan view, the thin plate portion has a width along a straight line along the first direction from the optical axis of the one first distal-end light source toward the outer edge greater than a width along a straight line from the optical axis of the second distal-end light source toward the outer edge.