A surface light source device includes a substrate, a plurality of light emitting devices and a light diffusion plate. Each light emitting device includes a light emitting element and a light flux controlling member. A cross section, perpendicular to the central axis of the light flux controlling member, of the emission surface is line-symmetric with respect to the X-axis and the Y-axis, and the cross section has the shortest length along the X-axis and the longest length along the Y-axis. For the light flux controlling member of one of the light emitting devices that is disposed closest to a corner of the light diffusion plate in plan view, a part having a largest curvature in the emission surface on the cross section of the emission surface is located closer to the X-axis than an intersection point of a light beam with the emission surface is.