We have established a new platform to control the rate of spontaneous emission (SE) of organic molecules in the visible-light region using a combination of a two-dimensional (2D) photonic crystal (PC) slab made of TiO2 and a single-molecule measurement method. The SE from single molecules of a perylenediimide derivative was effectively inhibited via a radiation field controlled by the 2D PC slab, which has a photonic band gap (PBG) for transverse-electric (TE)-polarized light. The fluorescence lifetimes of the single molecules were extended up to 5.5 times (28.6 ns) by the PBG effect. This result appears to be the first demonstration of drastic lifetime elongation for single molecules due to a PBG effect.