A signboard apparatus comprising: a substrate (2, 2A, 2B) shaped as a rectangular plate when viewed from a front side thereof; a strip-like light source (3, 3A, 3B) mounted on at least one out of both ends in a width direction of the substrate (2, 2A, 2B); and a light guide plate which is disposed across a front surface of the substrate (2, 2A, 2B), is mounted on the substrate (2, 2A, 2B), and guides light emitted from the strip-like light source (3, 3A, 3B) to every part in the width direction of the substrate (2, 2A, 2B); wherein the signboard apparatus is configured so that a display medium is capable of being disposed on a front surface side of the light guide plate, the substrate (2, 2A, 2B) includes: a main member; and a pair of light source mounts (20, 20A, 20B), which are formed separately from the main member and which are capable of having the strip-like light source (3, 3A, 3B) mounted thereupon, the main member includes: a main body that is shaped as a rectangular plate when viewed from a front side thereof; and a pair of edge plates that are provided at both ends in a width direction of the main body and cover the front surface side of both ends in a width direction of the light guide plate, both ends of the main body and both edge plates form a pair of insertion channels in a longitudinal direction of the main member into which the light guide plate is capable of being inserted, mounting convex portions for attaching the light source mounts (20, 20A, 20B) are formed on an inner surface of each of the edge plates, the light source mounts (20, 20A, 20B) include mounting concave portions into which the mounting convex portions can fit, and which are fixed to the respective edge plates by being inserted into both of the insertion channels with the mounting convex portions being fitted into the mounting concave portions, and the light guide plate is mounted on the substrate (2, 2A, 2B) in a state where both ends in the width direction have been inserted inside the insertion channels in a state where light from the strip-like light source (3, 3A, 3B) mounted on the light source mounts (20, 20A, 20B) is incident from end surfaces in the width direction of the light guide plate. a substrate (2, 2A, 2B) shaped as a rectangular plate when viewed from a front side thereof; a strip-like light source (3, 3A, 3B) mounted on at least one out of both ends in a width direction of the substrate (2, 2A, 2B); and a light guide plate which is disposed across a front surface of the substrate (2, 2A, 2B), is mounted on the substrate (2, 2A, 2B), and guides light emitted from the strip-like light source (3, 3A, 3B) to every part in the width direction of the substrate (2, 2A, 2B); wherein the signboard apparatus is configured so that a display medium is capable of being disposed on a front surface side of the light guide plate, the substrate (2, 2A, 2B) includes: a main member; and a pair of light source mounts (20, 20A, 20B), which are formed separately from the main member and which are capable of having the strip-like light source (3, 3A, 3B) mounted thereupon, a main member; and a pair of light source mounts (20, 20A, 20B), which are formed separately from the main member and which are capable of having the strip-like light source (3, 3A, 3B) mounted thereupon, the main member includes: a main body that is shaped as a rectangular plate when viewed from a front side thereof; and a pair of edge plates that are provided at both ends in a width direction of the main body and cover the front surface side of both ends in a width direction of the light guide plate, a main body that is shaped as a rectangular plate when viewed from a front side thereof; and a pair of edge plates that are provided at both ends in a width direction of the main body and cover the front surface side of both ends in a width direction of the light guide plate, both ends of the main body and both edge plates form a pair of insertion channels in a longitudinal direction of the main member into which the light guide plate is capable of being inserted, mounting convex portions for attaching the light source mounts (20, 20A, 20B) are formed on an inner surface of each of the edge plates, the light source mounts (20, 20A, 20B) include mounting concave portions into which the mounting convex portions can fit, and which are fixed to the respective edge plates by being inserted into both of the insertion channels with the mounting convex portions being fitted into the mounting concave portions, and the light guide plate is mounted on the substrate (2, 2A, 2B) in a state where both ends in the width direction have been inserted inside the insertion channels in a state where light from the strip-like light source (3, 3A, 3B) mounted on the light source mounts (20, 20A, 20B) is incident from end surfaces in the width direction of the light guide plate.