Testing device, and testing method for the line and one sheet using the testing device

A test device for a display device including a plurality of demultiplexing switches connected to a plurality of data lines in accordance with the present invention includes: a one-sheet test device configured to include a plurality of control switches connected to the demultiplexing switches through a plurality of wires; and a wire test device configured to transmit wire test signals for detecting defects in the wires to a pad connected to the control switches. The wire test device transmits the wire test signals to the pad to detect defects in first wires of the wires and then detect defects in remaining second wires thereof, and the first wires and the second wires are alternatively disposed below the demultiplexing switches to constitute paths for signals transmitted to the demultiplexing switches.