The conductivity of polydiacetylene thin films has been evaluated in the region below 20 mum using a newly constructed independently driven double-tip scanning tunneling microscope. The polydiacetylene thin films fabricated by means of the Langmuir-Blodgett method showed the formation of islands with sizes of 2-20 mum with the polydiacetylene backbone extended in one direction in each island. It was indicated that the resistance of the polydiacetylene thin films is proportional to the tip-tip distances, suggesting one-dimensional conduction along the polydiacetylene backbones, which was clearly different from that of two-dimensionally uniform conductive thin films of poly(3-octylthiophene). The conductivity of the polydiacetylene thin film was estimated to be (3-5) x 10(-6) S/cm, which is 5 orders of magnitude higher than that in the previous report.