4,4'-methylenebis{N-(3,5-di-tert-butylsalicylidene)-2,6-dimethylaniline} formed a definite acetone-inclusion crystal by recrystallization with acetone. Through crystal-to-crystal solvent exchange of the crystal, various types of crystals were obtained. All resulting crystals exhibited photochromism. Rate constants of thermal fading of photochromes were found to depend on the number of solvent molecules included in the crystal unit. The thermal stability of photochrome is discussed in connection with molecular mobility in the crystal lattice.