An aquarium overflow device is capable of preventing inadvertent supply of water in an aquarium from being supplied to an outside during, e.g., power failure. An aquarium overflow device is for supplying the water in an aquarium to an external device outside the aquarium. The device is equipped with a reversed U-shaped siphon pipe for supplying the water in the aquarium to an outside of the aquarium, the siphon pipe having an inlet side end portion disposed in the aquarium and an inlet side end portion disposed with the outlet side end portion disposed outside the aquarium, a suction device for sucking an inside of the siphon pipe, the suction device being connected to an upper portion of the siphon pipe, and an air introduction device for introducing air to an upper portion in the siphon pipe when suction by the suction device stops.
