A thermal printer includes a rotatable platen roller; an elongated thermal head which is movable toward or away from the platen roller; and a head supporting member having an elongated head supporting portion for supporting the thermal head formed at an end. The field effect transistor according to the present invention transistor contains at least an organic semiconductor layer, wherein the organic semiconductor layer contains at least a porphyrin compound and has a maximum diffraction intensity I1 in a Bragg angle range of 9. The demarcation lines form block-like geometry.