The present invention relates to a crystal of 1-ethyl-7-methyl-3-{4-[(3-methyl-3H-imidazo[4,5-b]pyridin-2-yl)oxy]phenyl}-1,3-dihydro-2H-imidazo[4,5-b]pyridin-2-one useful as a prophylactic or therapeutic agent for schizophrenia and the like, which shows an X-ray powder diffraction pattern having characteristic peaks at interplaner spacings (d) of 13.59 plus or minus 0.2 and 6.76 plus or minus 0.2 Angstroms in powder X-ray diffraction.
