Method for fabricating a semiconductor device and a substrate processing apparatus

A semiconductor device fabricating method includes the steps of loading one or more substrates into a boat disposed in a waiting room positioned next to a reaction furnace; vacuum-evacuating the waiting room to a vacuum state at a base pressure; loading the boat into the reaction furnace at a first ambient pressure; and recovering a temperature of the reaction furnace at a second ambient pressure. The first or the second ambient pressure is greater than the vacuum state but less than the atmospheric pressure. Further, the method includes the step of increasing the temperature of the one or more substrates at a third ambient pressure, and also the third ambient pressure is greater than the base pressure but less than the atmospheric pressure.