A test mark is provided, for use in an inspection after a display panel, TAB tapes, and a flexible board have been connected together. Defective panels can be rejected with high accuracy in the simple inspection, thereby mitigating the burden on the microscopic inspection. At a predetermined distance from each of connection areas for connecting a panel substrate, TAB tapes, and a flexible board, there are provided test marks with at least one on each component. The electrical connection is tested between each pair of the test marks to thereby determine whether the interconnection is in a good state.