An automatic water supply device comprises a pot-shaped case having a space enclosing a flowerpot in the inside, a supporting bed of reverse bowl shape enclosed in the case and holding the flowerpot thereon, a control unit arranged in a gap between the inside of the case and the flowerpot and containing at least an air pump, a controller and a timer, an air tube connected to outlet side of the air pump and having a front end part communicating with the inside of the supporting bed, and a device to exhaust air in the supporting bed when the supplying water is finished. Since the water supplying device basically has simple structure that a supporting bed of reverse bowl shape is inserted from the upper side and enclosed in the case, a part storing water can be taken to pieces and inspection or cleaning is easy. Also since the control unit is arranged between the inside of the case and the flowerpot, it can be easily detached and when the casing of the unit is made the waterproof structure, it can be used in enclosing in any position in the case without trouble. Further since all parts including the control unit are enclosed in the case, mechanical parts are not exposed or projected and good appearance can be secured. Also noise of the air pump can be prevented from being dispersed to the outside