Provided is a stable pharmaceutical composition for oral administration comprising 6-ethyl-3-({3-methoxy-4-[4-(4-methylpiperazin-1-yl)piperidin-1-yl]phenyl}amino)-5-(tetrahydro-2H-pyran-4-ylamino)pyrazine-2-carboxamide (hereinafter referred to as compound A) or a pharmaceutically acceptable salt thereof, wherein the generation of related substances during storage is inhibited. In the stable pharmaceutical composition for oral administration, the proportion of crystals of compound A or a pharmaceutically acceptable salt thereof is 60% or more with respect to the total amount of compound A or a pharmaceutically acceptable salt thereof.