Transient absorption spectra of the layered compound K4Nb6O17, which is known as a photocatalyst for splitting water into H-2 and O-2, were measured by the femtosecond UV pump and visible probe technique. Under band-gap excitation of a K4Nb6O17 single crystal and a spin-coated K4Nb6O17 film, a broad transient absorption extending over the whole visible region, which could be ascribed to photogenerated carriers in K4Nb6O17, was observed, and its decay behavior was analyzed on the basis of second-order kinetics. Dynamics of photoexcited Ru(bpy)(3)(2+) intercalated at K4Nb6O17 interlayers was also investigated. Intercalation of Ru(bpy)(3)(2+) in K4Nb6O17 is known to give visible light response for photocatalytic chemical reaction. Observed transient bleaching of Ru(bpy)(3)(2+) band showed fast and nonexponential decay differing from that of Ru(bpy)(3)(2+) in water. The decay mechanism is discussed in terms of electron transfer between Ru(bpy)(3)(2+) and K4Nb6O17.