According to one embodiment, a flat-panel display device includes a first substrate including an output pad to which a signal necessary for displaying an image on an active area is supplied from a signal supply source, a dummy pad juxtaposed with the output pad, a signal line connected to the output pad, a switching element connected to the signal line, a pixel electrode connected to the switching element in the active area, a first dummy line connected to the dummy pad, spaced apart from the signal line and including a first projection protruding in a direction crossing a direction of extension of the first dummy line, and a second dummy line spaced apart from the dummy pad and the signal line and including a second projection opposed to the first projection.