[Object] To provide a method of producing a thin film transistor superior in productivity and capable of preventing variation in transistor characteristics among devices from occurring to improve carrier mobility, and a thin film transistor.[Solving Means] In a method of producing a thin-film transistor according to the present invention, a solid-state green laser is irradiated onto a channel portion of an amorphous silicon film using a source electrode film and a drain electrode film as masks, thereby improving mobility. Since the channel portion of the amorphous silicon film is crystallized by the irradiation of the solid-state green laser, laser oscillation characteristics can be more stable than in a conventional method that uses an excimer laser. Further, laser irradiation onto a large-size substrate at uniform output characteristics in plane becomes possible, with the result that a variation in crystallinity of channel portions among devices can be avoided. Moreover, since a maintenance cycle of a laser oscillator becomes longer, a downtime cost of the apparatus can be reduced and productivity can be improved.