A photosensor including a semiconductor thin film having a light receiving portion includes the following elements. A substrate has a recess with an inclined side wall having a forward tapered shape. A reflective material layer is disposed along the recess of the substrate. An insulating layer covers the substrate having thereon the reflective material layer. The semiconductor thin film is disposed on the insulating layer so as to cross the recess. The light receiving portion of the semiconductor thin film is disposed above the recess.