A display device includes a substrate, a circuit layer, a light emitting element layer, and a display driving circuit. The circuit layer comprises demux circuit units side by side in a demux area of a non-display area of the substrate and electrically connected between data lines and the display driving circuit, test signal supply lines in the non-display area and transmitting test signals for testing the lighting state of light emitting elements of the light emitting element layer, and test pad connection lines respectively electrically connected to test signal pads in the sub-area and extending to the non-display area. The test signal supply lines are electrically connected to the test pad connection lines through test line connection contact holes, respectively. The test line connection contact holes are in a test connection area which is a part of the demux area adjacent to the sub-area.