A water closet has a variable siphon soil exhaust system with a flexible water bladder connected to a water inlet, a tension spring within the water bladder to contract the water bladder, a variable siphon tube connected to the water bladder to allow soil to be exhausted from the bowl while lifted and lowered by an expanding and contracting motion of the water bladder, and a small tube for connecting the water bladder to the variable siphon tube. A cover has a first port connected to the water supply passage, a second port connected to the bowl, and a third port connected to a bottom tube laid under the floor. The variable siphon tube has one end connected to the bowl through a bellows tube and an opposite freely movable open end communicating with the interior of the cover.