DESCRIZIONE
An overflow device for a water tank, having a simplified structure and whose cost is reduced. The overflow device for a water tank supplies water (W) in the water tank to an external apparatus outside the water tank and supplies water (W) of the external apparatus into the water tank. The overflow device has a reverse U-shaped siphon pipe (55) having the inflow side end placed inside the water tank, having the outflow side end placed outside the water tank, and leading the water (W) in the water tank to supply it to the external apparatus; a reverse U-shaped discharge pipe (56) having the inflow side end placed outside the water tank and also having the outflow side end placed inside the water tank; water sending means for sending the water (W) out of the external apparatus and causing it to flow into the discharge pipe (56); and a connection tube (8) for interconnecting the upper part of the siphon pipe (55) and the upper part of the discharge pipe (56). When the water (W) is supplied to the discharge pipe (56) by drive of the water sending means, the inside of the siphon pipe (55) is sucked via the connection tube (8) by suction force produced by a flow of the water (W).