There are provided a flush water tank apparatus capable of accurately setting the amount of flush water to be discharged while opening a discharge valve by a discharge valve hydraulic drive unit. A flush water tank apparatus includes a valve controller, where, in a case where a first amount of flush water is selected, the valve controller causes the timing control mechanism to be engaged with the discharge valve and causes the timing control mechanism to operate such that engagement is released after a lapse of a first period of time, and in a case where a second amount of flush water is selected, the valve controller causes the timing control mechanism to be engaged with the discharge valve and causes the timing control mechanism to operate such that engagement is released after lapse of a second period of time shorter than the first period of time.