We fabricated polyvinyl chloride (PVC) films containing 2′-anilino-6′-di-n-pentylamino-3′-methylfluoran (Black305) to develop radiochromic materials with high sensitivity to radiation. The optical absorption spectra of these films before and after X-ray irradiation were measured, and for comparison, we also investigated the absorption spectra of polyvinyl butyral, poly(methyl methacrylate), and polystyrene films containing Black305. The absorbance at 460 and 600 nm increased with the X-ray dose, and the change in the absorbance of the PVC film integrated over wavelengths 350-800 nm was 3.2 × 102-8.1 × 102 times those of other films at a radiation dose of 0.5 kGy. This suggests that the oxidation of Black305 proceeds efficiently by radicals generated from PVC. The sensitivity to radiation of the PVC film was 4.4-51 times greater than that of conventional plastics containing leuco dyes, and the lower limit of detection of 1 mm PVC film containing 1 wt% Black305 was 0.11 Gy, suitable for radiotherapy dosimetry.