Abstract: The water saving siphon cover 100 comprises a flexible air chamber 10 and a rigid air housing 20 connected to the lower side of the air chamber 10 to form an air room 30 inside. The air chamber 10 comprises a supporting portion 11, a variable portion 12 having a horizontal portion 12a and a sloping portion 12b, a ceiling portion 13 closing the upper side of the variable portion 12, an outer flange 15, and a pair of hinge portions 17, each being extended from one side of the outer flange 15. The cup shaped air housing 20 comprises a suction hole 22 and an air vent 23. The water saving siphon cover 100 is installed such that the end of the hinge portion 17 blocks an outlet port 36 in a water tank 37, being hinged to the overflow tube 31. Thus, the water saving siphon cover 100 can save water according to the required amount of wash water based on a change in the opening time of the outlet port 36 caused by a difference between the buoyant force for flushing urine and the buoyant force for flushing feces.