Since sodium contained in glass, or glass itself has low heat resistance; a CPU fabricated using a TFT formed over a glass substrate or the like has not been obtained. In the case of operating a CPU with high-speed, the length of a gate (gate length) of a TFT is required to be shorter. However, since a glass substrate has large deflection, a gate electrode cannot have been etched to have a gate length short enough to be used for a CPU. According to the invention, a conductive film is formed over a crystalline semiconductor film formed over a glass substrate, a mask is formed over the conductive film, and the conductive film is etched by using the mask; thus, a thin film transistor with a gate length of 1.0 μm or less is formed. In particular, the crystalline semiconductor film is formed by crystallizing an amorphous semiconductor film formed over a glass substrate by laser irradiation.