A method for producing a thin film semiconductor device is described. In the method, a thin film layer of non-single-crystalline semiconductor, which is deposited on a base layer of glass, is processed to an island-shaped thin film layer at the time prior to the layer irradiation step. The laser irradiation to the thin film layer of non-single-crystalline semiconductor is carried out after forming an insulation film layer and a gate electrode over the island-shaped thin film layer, by using the gate electrode as the irradiation mask, whereby the center area of the island-shaped thin film layer masked by the gate electrode is crystallized, and simultaneously, the both side areas thereof which is not masked by the gate electrode are annealed. Next, a source electrode and a drain electrode is formed in the annealed areas. The implantation of impurity ion may be carried out either before or after the laser irradiation. By the above order of steps, it becomes possible to obtain a thin film semiconductor device which has small numbers of crystals and less variance of grain size of crystals for each unit of electric circuit, compared with a device produced by the conventional process. Further, the process makes it possible to be even the boundary surface between the crystallized layer and the insulation layer in a device.