To improve the rotary efficiency of an impeller installed in a water passage, increase a generated electric power by one water supply, depress pressure loss in the impeller, improve durability of the impeller, in an automatic water supply device detecting users, automatically supplying water provided with an electric generator generating electric power by water supply.

The automatic water supply apparatus having a human body detector detecting presence/absence of users, an electromagnetic valve electrically opening/closing the water passage, an electric power generator provided with the impeller in the water passage, a battery storing electric power generated by the electric power generator, and a controller controlling these parts, the apparatus further comprises a swirling means for swirling the flow of supply water in the upstream side of the impeller and applying the rotary force of the supply water generated by the circulation to an inclined part of the impeller, and a first water passage of the supply water at the central side of the impeller and a second water passage at the outer peripheral side to diverge the supply water. Moreover, a homopolar magnet is arranged in the bearing holding the lower end part and the rotary shaft of the impeller.