An active matrix substrate includes a plurality of transistors. A source electrode is connected with a data signal line, and a drain electrode is connected with a pixel electrode in each transistor. The source electrode is located on a semiconductor layer, and at least a portion of the drain electrode is overlapped with the gate electrode. A gate insulating film covering the gate electrode of each transistor has a thin section having a reduced film thickness, at a portion where the gate insulating film is overlapped with each gate electrode. An overlapping area of the thin section with the source electrode is smaller than an overlapping area of the thin section with the drain electrode. Thus, the active matrix substrate can prevent the generation of short-circuits between the signal lines (between the data signal line and a scanning signal line) in a TFT forming region, while guaranteeing TFT characteristics.