The mechanical characteristics of the support substrate and the insulation layer of an organic field effect transistor are optimized to thereby attain a high mobility, a high on-current and a low leakcurrent, and a high on/off ratio in the organic field effect transistor. A field effect transistor comprising, provided on a support substrate (1), an insulation layer (3), a gate electrode (2) and an organic semiconductor layer (4) separated from this insulation layer (3), and a source electrode (5) and a drain electrode (6) provided so as to contact the organic semiconductor layer (4). Elongation [epsilon]1 (%) at a yield point of the insulation layer (3) is larger than elongation [epsilon]2 (%) at a yield point of the support substrate (1). Good flexible characteristics provided can restrict cracks that might be caused by a stress applied to reduce a leak current, whereby a high mobility, a high on-current and a low leak current, and a high on/off ratio are attained.