To provide an automatic water supply device surely operable by manual operation even if water pressure of a pipeline is low.

This automatic water supply device has an inlet passage 3, a hood 14 connected to a diaphragm 12, forming a primary pressure chamber 18 in an upper part of the diaphragm by opening-closing an opening part 6 of the inlet passage, and installing a drain valve 19 communicated with the primary pressure chamber, a water introducing pipe 31 having one end opening in the upstream side inlet passage of the opening part, and having the other end opening in the primary pressure chamber, an atmospheric opening valve 41 for opening the water introducing pipe in the atmosphere at valve opening time, and cutting off the opening at valve closing time by interposing in the middle of the water introducing pipe, and a water level sensor 35 for closing the atmospheric opening valve at an upper limit water level of control object water, and opening the atmospheric opening valve at a lower limit water level by interlocking with a water level change in the control object water.