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

The present invention provides a flush water tank device (4) that supplies flush water to an upper and lower spout port of a flush toilet (2) including; a flush water tank (10); a discharge valve (12); a first on-off valve (19) that performs switching between ejection and stopping of the flush water from the upper spout port; a first on-off valve driving mechanism (16) that is moved from a stopped position to an ejection position based on an user operation; a biasing mechanism (17) that causes the first on-off valve driving mechanism to move to the stopped position; a holding mechanism (20) that holds, at the ejection position, the first on-off valve driving mechanism at the ejection position against the bias force; and a releasing mechanism (21) that releases the holding of the first on-off valve driving mechanism, using the flush water flowing out from the first on-off valve.