This semiconductor device (100) includes a substrate (1), a gate electrode (11), a gate insulating film (12), an oxide semiconductor layer (13), a source electrode (14), a drain electrode (15), and a protective film (16). The upper and side surfaces of the oxide semiconductor layer are covered with the source and drain electrodes and the protective film. When viewed along a normal to the substrate, the narrowest gap between the respective outer peripheries of a first contact region (13s) and the source electrode and the narrowest gap between the respective outer peripheries of a second contact region (13d) and the drain electrode both have a length of 1.5 μm to 4.5 μm.