　　Photochromism of diarylethenes was observed at the single-molecule level. The open-ring form of the molecule A (fluorescent) converts to the closed-ring form B (non-fluorescent) opon irradiation with ultraviolet light light, and the B from to A form upon irradiation with visible light; both forms are thermally stable. The process was followed by the fluorescence change. Initially, the film containing nonfluorescent closed-ring form molecules was dark, but when irradiated with visible light for 10 s, for single molecules switched to the fluorescent 'on' state. Upon irradiation with weak ultraviolet light for 3 s, the fluorescent molecules then switched to the non-fluorescent 'off' state. The fluorescent state was restored by irradiating with visible light for 10 s. This digital reversible switching is due to photochromism of the diarylethene unit at the single-molecule level.