The photoconversion of a crystalline-phase precursor, 6,13-dihydro-6,13-ethanopentacene-15,16-dione (PDK), into pentacene was examined. Upon irradiation with 470 nm light, the photoconversion proceeded even in the crystalline phase. Although the rate of the crystalline phase photoconversion was much slower than that in solution, quantitative photoconversion was achieved upon lengthy photoirradiation. The photoconversion of PDK was also confirmed at the single μm-sized crystal level by observing fluorescence images and spectra of the PDK crystal using confocal microscopy. Destruction of the crystal was observed when some of the PDK molecules in the crystal were photoconverted into pentacene. These results indicate that the crystalline-phase photoconversion of PDK proceeded through the destruction of the crystal. Quantitative photoconversion was achieved as a result.