To make it possible to efficiently generate a siphoning action with a small quantity of water by providing a water spraying section for supplying cleaning water from upper section of a water discharge passage to the inner wall of a downstream side water discharge passage, in a trap water discharge passage. A control device 15 opens a rim valve in an automatic two-way selector valve 14 in accordance with a signal from a manipulating section 16 so as to feed cleaning water into a bowl section 3 of a closet 1 which is therefore subjected to precleaning. When a detection value by flowmeter 19 coincides with a set value of precleaning water quantity, the control device 15 closes the rim valve. Then the control device 15 opens a shower valve in the selector valve 14 so as to jet water from a shower unit 7 into a flow-out unit 4c in a trap draining passage 4. Accordingly, the cleaning water discharges air into a water discharge pipe from a flow-out port 6 while it swallows up air in an flow-out passage 4c so as to generate a negative pressure. Further, turbid water 23 in the bowl section 3 overflows a weir 4b in the water discharge passage 4 and is led into a flow-out passage 4c so that the negative pressure is increased, thereby a siphoning action generates. |