A semiconductor device, a thin film transistor, and a method for producing the same capable of decreasing the management cost, and capable of decreasing the production steps to reduce the production cost are proposed. A method for producing a thin film transistor 2 provided with a semiconductor which is composed of a prescribed material and serves as an active layer 41 and a conductor which is composed of a material having the same composition as that of the prescribed material and serves as at least one of a source electrode 51, a drain electrode 53 and a pixel electrode 55, which includes the steps of simultaneously forming into a film an object to be processed and a conductor (a source electrode 51, a source wire 52, a drain electrode 53, a drain wire 54 and a pixel electrode 55) which are composed of the amorphous prescribed material, followed by simultaneous shaping, and crystallizing the object to be processed which has been shaped to allow it to be the active layer 41.