A toilet bowl flush tank which comprises a large flush valve and a small flush valve for switching over flush water supplied to a toilet bowl between a large amount and a small amount. The large flush valve (A) and the small flush valve (B) are coaxially arranged, and buoyancy imparted to valve discs (5, 7) is used for detection of valve closing water levels, so that closing of the flush valves (A, B) are controlled.