An overflow device for a water tank, capable of preventing water in the tank from being unexpectedly supplied to the outside of the tank in an electric power failure etc. The overflow device supplies water in the water tank to an external device outside the tank. The overflow device has a reverse U-shaped siphon pipe (55) having the inflow side end placed in the water tank, having the outflow side end placed outside the tank, and supplying the water in the tank to the outside; suction means connected to the upper part of the siphon pipe (55) and sucking the inside of the suction pipe; and airintroduction means for introducing air to the upper part in the siphon pipe when the suction by the suction means is stopped.