A flush water tank apparatus (4, 104) for supplying flush water to a flush toilet, the flush water tank apparatus comprising: a storage tank which stores flush water to be supplied to the flush toilet and in which a drain port for discharging the stored flush water to the flush toilet is formed; a discharge valve that opens and closes the drain port and that supplies flush water and stops the supply of flush water to the flush toilet; a discharge valve hydraulic drive unit (14, 114) that drives the discharge valve by using water supply pressure of supplied tap water; a clutch mechanism (30, 130) that couples the discharge valve and the discharge valve hydraulic drive unit to pull up the discharge valve by a driving force of the discharge valve hydraulic drive unit and that is disconnected at a predetermined pull-up height of the discharge valve to make the discharge valve descend; a flush water amount selection portion capable of selecting between a first amount of flush water for flushing the flush toilet and a second amount of flush water that is different from the first amount of flush water; a first float device that is moved according to a water level in the storage tank, the first float device being configured to be switched according to the water level from a holding state in which descent of the discharge valve is prevented to a non-holding state in which the descent is not prevented so as to discharge the first amount of flush water; a second float device that is moved according to the water level in the storage tank, the second float device being configured to be switched from a holding state in which descent of the discharge valve is prevented to a non-holding state in which the descent is not prevented, according to the water level so as to discharge the second amount of flush water; and an adjustment mechanism (58, 158) that adjusts the pull-up height of the discharge valve with which the clutch mechanism is disconnected, the adjustment mechanism being configured so that when the second amount of flush water is selected by the flush water amount selection portion the clutch mechanism is disconnected at a pull-up height of the discharge valve such that the discharge valve descended by the disconnection of the clutch mechanism is held by the second float device in the holding state. a storage tank which stores flush water to be supplied to the flush toilet and in which a drain port for discharging the stored flush water to the flush toilet is formed; a discharge valve that opens and closes the drain port and that supplies flush water and stops the supply of flush water to the flush toilet; a discharge valve hydraulic drive unit (14, 114) that drives the discharge valve by using water supply pressure of supplied tap water; a clutch mechanism (30, 130) that couples the discharge valve and the discharge valve hydraulic drive unit to pull up the discharge valve by a driving force of the discharge valve hydraulic drive unit and that is disconnected at a predetermined pull-up height of the discharge valve to make the discharge valve descend; a flush water amount selection portion capable of selecting between a first amount of flush water for flushing the flush toilet and a second amount of flush water that is different from the first amount of flush water; a first float device that is moved according to a water level in the storage tank, the first float device being configured to be switched according to the water level from a holding state in which descent of the discharge valve is prevented to a non-holding state in which the descent is not prevented so as to discharge the first amount of flush water; a second float device that is moved according to the water level in the storage tank, the second float device being configured to be switched from a holding state in which descent of the discharge valve is prevented to a non-holding state in which the descent is not prevented, according to the water level so as to discharge the second amount of flush water; and an adjustment mechanism (58, 158) that adjusts the pull-up height of the discharge valve with which the clutch mechanism is disconnected, the adjustment mechanism being configured so that when the second amount of flush water is selected by the flush water amount selection portion the clutch mechanism is disconnected at a pull-up height of the discharge valve such that the discharge valve descended by the disconnection of the clutch mechanism is held by the second float device in the holding state.