This planar light source device has a pair of substrates, a pair of irradiation plates disposed between the pair of substrates, and a plurality of light emission devices disposed on one or both of the pair of substrates. In a cross-section of the light emission devices in a direction perpendicular to the irradiation plates through an optical axis, the luminosity of light emitted in a 7.0° direction when 0° is the optical axis direction and the luminosity of light emitted in a (tan−1(t/L))° (where t represents the gap between the irradiation plates, and L represents the gap, in a direction along the pair of irradiation plates, from the surface of a light emission device disposed on one of the substrates to an end part of the other-substrate-side irradiation plate) direction or a (tan−1(t/2L))° direction satisfy a prescribed relationship.