Provided is a stable pharmaceutical composition for oral administration containing 6-ethyl-3-({3-methoxy-4-[4-(4-methylpiperazin-1-yl)piperidine-1-yl]phenyl}amino)-5-(tetrahydro-2H-pyran-4-ylamino)pyrazine-2-carboxamide (compound A hereinafter), or a pharmacologically acceptable salt thereof, that suppresses the generation of related substances during storage. The proportion of crystals of the compound A or pharmacologically acceptable salt thereof in the stable pharmaceutical composition for oral administration is 60% or higher relative to the total weight of the compound A or pharmacologically acceptable salt thereof.
