It was unexpectedly observed that the volume electrical resistivity of UV-curable polymer film including a small amount of vapor-grown carbon fiber (VGCF) along thickness direction decreased by three ordered magnitude (from 1010 to 107 Ω· cm) after UV irradiation. The cause of the rapid decrease was investigated by changing various parameters, such as, VGCF concentration, thickness, VGCF structure (linear or branch, dispersion), and volume shrinkage of UV-curable polymer matrix. The decrease occurred reproducibly when the concentration of VGCF was between 0.005 and 0.1 wt % with thickness less than 25 μm. The moving distance of VGCF by UV irradiation through the volume shrinkage of the matrix was confirmed to be around 9.0 micron at the center part of the film. It was suggested from optical microscope observation that the presence of a trace amount of branched VGCF (L, X, Y form) having similar length to the film thickness moved by the volume shrinkage of UV-curable polymer matrix during UV irradiation and made the conductive paths along thickness direction.