PURPOSE: To sharply shorten the drying time by sealing a room, discharging the heated steam, the condensed water, and the gas contained in the heated steam in the room, then raising the room temperature with the air dried by a cold trap for drying.

CONSTITUTION: When a room 12 is to be sterilized, heated steam is fed into the room 12 from a boiler 1. When the set sterilization time elapses after a thermometer detects that the room 12 reaches the sterilization temperature, valves 2, 3 are closed, and the sterilization work is completed. A cold trap 18 and a vacuum pump 20 are operated, the heated steam and the condensed water are recovered, and the room 12 is quickly dried. When the air not recovered by the cold trap 18 is discharged, the room 12 is made to be the vacuum state, and the condensed water stuck on the wall is quickly evaporated. Air is cooled by the cold trap 28, it is sterilized by a heater 29 after moisture is removed, and the dry air cooled by a cooler 30 is guided into the room 12 via a valve 6 to complete drying.