Problem: To provide a flush water tank apparatus capable of stable operational control of a discharge valve apparatus, and stable flush water supply to a toilet for long term.Solution Means: The flush water tank apparatus of the present invention includes: a power drive unit comprising a motor 60 and an output shaft 62, being a power drive unit 54 capable of electrically operating a drain valve on a drain valve mechanism; and a housing portion 56 formed so as to surround the perimeter of the outlet on a drive unit internal air passage 70 extending from the motor in the interior of the flush water tank; wherein an air passage communicating from the motor 60 of the power drive unit 54 through the housing portion 56 to the exterior of the flush water tank 8 is formed.