The flush water tank apparatus of the present invention includes: a clutch mechanism 30 coupling the discharge valve 12 and the discharge valve hydraulic drive unit 14 to pull up the discharge valve, and being disconnected at a predetermined timing to cause the discharge valve to descend; flush water amount selection device capable of selecting from a first and a second amounts of flush water; a float device 26 including a float and a holding mechanism switchable between a holding and a non-holding states in conjunction with movement of the float; and a timing control mechanism controlling a timing of a drain port 10a being blocked; and, when the second amount is selected, causes the second amount of flush water to be discharged by switching the holding mechanism to the non-holding state before a water level in a storage tank drops to a predetermined water level.