A thin film transistor according to the present disclosure including: a gate electrode above a substrate; a gate insulating layer covering the gate electrode; a semiconductor layer above the gate insulating layer; and a source electrode and a drain electrode which are above the gate insulating layer, and electrically connected to the semiconductor layer, in which the gate insulating layer includes a first area and a second area, the first area being above the gate electrode, the second area being different from an area above the gate electrode, and made of a same substance as the first area, and the first area has a higher density than a density of the second area.