A planar light source device, comprising: a light source module; a light guide plate having a light introduction surface into which light from the light source module is introduced, and a light emission surface which has a larger surface area than the light introduction surface and from which light is emitted; and a stepped portion formed by a recessed part of the introduction surface of the light guide plate, wherein the light source module comprises: a circuit board; at least two point light sources, which are electrically connected to the circuit board and are arranged on an upper surface of the circuit board in a longitudinal direction of the circuit board; and a light guide for guiding light, which is introduced from the point light sources, in an upward direction of the circuit board, wherein the light guide comprises: a first reflective wall disposed on the upper surface of the circuit board; and a second reflective wall disposed on the upper surface of the circuit board such that a space, in which the point light sources are disposed, is defined between the first reflective wall and the second reflective wall, wherein the second reflective wall has a greater height than the first reflective wall, wherein the stepped portion mates with an upper portion of the second reflective wall. a light source module; a light guide plate having a light introduction surface into which light from the light source module is introduced, and a light emission surface which has a larger surface area than the light introduction surface and from which light is emitted; and a stepped portion formed by a recessed part of the introduction surface of the light guide plate, wherein the light source module comprises: a circuit board; at least two point light sources, which are electrically connected to the circuit board and are arranged on an upper surface of the circuit board in a longitudinal direction of the circuit board; and a light guide for guiding light, which is introduced from the point light sources, in an upward direction of the circuit board, a circuit board; at least two point light sources, which are electrically connected to the circuit board and are arranged on an upper surface of the circuit board in a longitudinal direction of the circuit board; and a light guide for guiding light, which is introduced from the point light sources, in an upward direction of the circuit board, wherein the light guide comprises: a first reflective wall disposed on the upper surface of the circuit board; and a second reflective wall disposed on the upper surface of the circuit board such that a space, in which the point light sources are disposed, is defined between the first reflective wall and the second reflective wall, wherein the second reflective wall has a greater height than the first reflective wall, wherein the stepped portion mates with an upper portion of the second reflective wall. a first reflective wall disposed on the upper surface of the circuit board; and a second reflective wall disposed on the upper surface of the circuit board such that a space, in which the point light sources are disposed, is defined between the first reflective wall and the second reflective wall, wherein the second reflective wall has a greater height than the first reflective wall, wherein the stepped portion mates with an upper portion of the second reflective wall.