Abstract: A culture medium used for cultivating mushrooms is sterilized by a sterilizing apparatus. The sterilizing apparatus comprises a sterilizing container in which the culture medium is placed, a steam supply pipe for supplying steam in the sterilizing container, an exhaust pipe for exhausting steam and air in the container, and a drain pipe for draining condensate from the container. The drain pipe includes a main pipe having a relatively large diameter and a bypass pipe provided with a trap. The amounts of steam supplying and exhausting and the amount of condensate draining are controlled in accordance with a predetermined mode during the sterilizing processes by regulating valve means mounted to the respective pipes. Two exhaust pipes having different exhausting capacities may be provided in consideration of environmental conditions.