A toilet stool having a supporting water tank for preventing overflowing

A toilet bowl having an auxiliary water tank for overflow prevention is provided to prevent cleaning water of a washbowl from overflowing to the outside when a one-piece toilet bowl gets clogged with sewage. A toilet bowl having an auxiliary water tank for overflow prevention includes a water tank(109) holding cleaning water at the appointed quantity using a ball tap(160) supplied with water to control water supply according to the water level of a water tank, a washbowl(118) having a rim waterway(122) and discharge ports(125) placed in the inner circumferential surface of a rim, and connected with the water tank by a side waterway(129), a water cover(113) placed on the lower part of the water tank to open and close the side waterway, an injection pump(150) inserted between the ball tap and the rim waterway to supply the mixed cleaning water to the rim waterway, and an auxiliary water tank(170) for overflow prevention installed to the inside of the water tank and provided with an injection pump(150), and an opening and a check valve(172) to open the flow of cleaning water in one direction.