Abstract: A drainage apparatus and a drainage method for a reactor coolant system are disclosed. According to the drainage apparatus and the drainage method, the gravity drainage procedure for performing the decomposition maintenance of a main valve forming the pressure boundary part between the RCS and the safety injection system is improved by a forced drainage procedure using a drainage pump, and there are the effects that working period for a planned preventive maintenance is shortened, liquid waste of a coolant of the reactor is not generated, reprocessing costs are decreased, and the economical efficiency and the safety of the nuclear power plant are improved.