A luminous keyboard device includes plural keys, a light-emitting element, and a light guide plate. By the light guide plate, a light beam emitted by the light-emitting element is guided to the plural keys. The light guide plate includes plural elongated slots and a light-absorbing part. The plural elongated slots are located near a lateral edge of the light guide plate. After the light beam transferred within the light guide plate is transmitted through the elongated slots and departed from the light guide plate, the fraction of the light beam introduced into the lateral edge of the light guide plate is reduced. The light-absorbing part is located at the lateral edge of the light guide plate for absorbing the portion of the light beam which is introduced into the lateral edge of the light guide plate. Consequently, the lateral light leakage of the light guide plate can be avoided.