The present invention provides a toilet bowl flushing water tank device which can largely open a discharge valve element instantaneously and, at the same time, can ensure a relatively large space in the vicinity of a discharge valve device thus enhancing a discharge capacity with a small power. The toilet bowl flushing water tank device includes a flushing water tank body forming a discharge opening in a center portion thereof, a discharge valve device for vertically moving a discharge valve element relative to the discharge opening, the discharge valve device being mounted and housed in the flushing water tank body, a leverage device being interlockingly connected to the discharge valve device, the leverage device being mounted and housed in the flushing water tank body, and a push button being interlockingly connected to the leverage device in the flushing water tank body and mounted on a tank lid body of the flushing water tank body. The push button is configured to be arranged at a position offset to one side from a position where the discharge opening is formed, the leverage device is constituted of a lever which is interlockingly connected to the discharge valve element and a pedestal which pivotally supports a middle portion of the lever. The pedestal pivotally supports a middle portion of the lever at a position closer to a push button side than the discharge valve device.