A flush water supply device includes: a water supply unit including a water supply valve; a water compartment fittingly attached to the water supply unit; a float provided inside the water compartment; and a swingable member having one end connected to the float and the other end connected to the water supply valve, wherein the water compartment is formed in a flattened shape in which a maximum vertical length becomes less than a maximum horizontal length by a given value, and a ratio of a distance between the fulcrum adjacent to the water supply valve and an effort point defined by the swingable member and the float, to a distance between the fulcrum adjacent to the water supply valve and a load point defined by the swingable member and the water supply valve is set to a given value or more.
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