Deposition mechanism and a diameter dependence of the conductivity of polythiophene derivative fibrils polymerized in pores of a membrane filter having submicrometer-sized pores aperture were discussed. Deposition process of the fibrils was able to be divided in the following three stages from SEM observation and polymerization current measurements. The first stage is the generation of deposition growth nucleus on the pore-inside wall and the filter surface, the second is growth of thickness of a thin-layer-deposit, and the third is the accumulation of a spherical deposit. And, when the pore size became big, the second stage shortened remarkably, and thickness of the thin-layer-deposit became thinner, and the spherical deposition began earlier. From the simulated conductivity of the fibrils with each diameter, at the smaller pore aperture than a specific value, the thickness of the thin-layer-deposit having high-conductivity increases. When the pore aperture becomes smaller, the thin-layer-deposition is remarkably easy to occur.