The authors have revealed that paper having high smoothness, small pore size, and low drying ability is suitable for discoloring decolorable toner. In the heat erasing process, water accelerates the dissociation of leuco dye and developer and/or the diffusion of developer, considering the high dielectric constant of water and the solubility of developer in water. The authors have confirmed diffusion of the developer into paper from the toner in the heat erasing process by the direct evidence and results of energy dispersive x ray and time-of-flight-secondary ion mass spectrometry.