A flush water supply device includes: a water supply valve for switching between a water supplying state and a water stopping state with respect of an inside of the flush water tank; a refill water system on a downstream side of the water supply valve and adapted to allow flush water to be supplied to the flush water tank, and further supplied to a toilet main body as refill water, wherein the refill water system includes a common passage-forming portion which forms a common passage having: an inlet port for allowing inflow of the flush water therethrough; a tank-side outlet port for allowing the flush water to flow out toward the flush water tank therethrough; a main unit-side outlet port for allowing the flush water to flow out toward the toilet main body therethrough; and a vent port; and a vacuum break valve in the common passage-forming portion.