In a portion of a gate signal line and a portion of a common signal line, cutouts which are arranged perpendicular to the extending direction of these lines and open to face each other in an opposed manner are formed. A cruciform shape in appearance is formed by combining a gap defined between the gate signal line and the common signal line extending parallel to each other and the cutouts to each other. The cruciform portion formed in this manner is used as an alignment mark in the exposure of a photolithography step of a layer formed later. Due to such a constitution, in manufacturing a thin film transistor substrate, it is possible to realize the highly accurate alignment without forming a pattern only used for alignment.