Provided is a pharmaceutical composition, which comprises 5-{[(3R)-1-acryloylpyrrolidin-3-yl]oxy}-6-ethyl-3-({4-[4- (4-methylpiperazin-1-yl)piperidin-1-yl]phenyl}amino)-pyrazine-2-carboxamide, or a pharmaceutically acceptable salt thereof, and is stabilized. The pharmaceutical composition comprises potassium chloride and/or sodium chloride, and inhibits the increase of related substances.本發明係提供一種包含5-{[(3R)-1-丙烯醯基吡咯啶-3-基]氧基}-6-乙基-3-({4-[4-(4-甲基哌嗪-1-基)哌啶-1-基]苯基}胺基)吡嗪-2-甲醯胺或其製藥學上可容許之鹽的安定的醫藥組成物。前述醫藥組成物係包含氯化鉀、及/或氯化鈉,可抑制類似物的增加。