PROBLEM TO BE SOLVED: To provide a water tank equipped with an automatic water supply device for automatically supplying water top a flowerpot for extended periods. SOLUTION: This water tank has a water tank chamber 1, drain hole 2 in the bottom of the chamber 1, and water stop valve element 3, which opens or shuts the drain hole 2. The water stop valve element 3 is opened or shut, in order to keep water level in the water container 8, by a vertical motion of the oscillating level 5 working in communication with the float 6, which vertically moves following water level in the container 8.
