PURPOSE: To obtain a sure sterilizing effect with an antiseptic solution prevented from decreasing its concentration by supplying the spare antiseptic solution into a washing tank, after washing work of an endoscope is finished, and displacing a water drop, which adheres to the endoscope, with the spare antiseptic solution and thereafter supplying the antiseptic solution.

CONSTITUTION: An endoscope 26 after use is placed on a table 13 with a washing process finished, thereafter at the time of successively supply-controlling an antiseptic solution, preliminary sterilization is at least once performed. That is, a spare antiseptic solution in an antiseptic solution tank 19 is supplied to a washing tank 12 through a water feed system pipe line 14. Following driving a feed liquid pump 23, the solution is supplied respectively to a rotary washing nozzle 24 and a channel connection port 25 through a circulating pipe line 15 to perform preliminary sterilization of the endoscope 26. Accordingly, a water drop, which adheres to the endoscope 26 or the like, is displaced with the spare antiseptic solution by this preliminary sterilization. A water supply valve 18 and a solenoid valve 21 are again open-operated, and the antiseptic solution in the antiseptic solution tank 19, in a condition of being mixed in supply water, is supplied into the washing tank 12 through the water feed system pipe line 14 with the main sterilizing process performed.