Flush water tank apparatus and flush toilet apparatus provided with the same

A flush water tank apparatus for supplying flush water to a flush toilet, the flush water tank apparatus comprising:

• a storage tank storing flush water to be supplied to the flush toilet, with a drain port for discharging the stored flush water to the flush toilet formed therein;
• a discharge valve opening/closing the drain port and performing supply/stop of the flush water to the flush toilet;
• a discharge valve hydraulic drive unit driving the discharge valve using water supply pressure of supplied tap water;
• a clutch mechanism coupling the discharge valve and the discharge valve hydraulic drive unit to pull up the discharge valve by driving force of the discharge valve hydraulic drive unit, and being disconnected at a predetermined timing to cause the discharge valve to descend;
• a flush water amount selection device capable of selecting between a first amount of flush water for washing the flush toilet and a second amount of flush water smaller than the first amount of flush water;
• and
• a timing control mechanism controlling, when the second amount of flush water is selected by the flush water amount selection device, a timing of causing the discharge valve to descend so that a timing of the drain port being blocked is earlier than a case of the first amount of flush water being selected.