A tank device for flushing a toilet bowl includes a tank body for storing toilet bowl-flushing water. The tank body has an opening at the bottom. One or more support arms are arranged within the tank body and are pivotally mounted to supports. A valve body is secured to one end of the support arms. The valve body comes into close contact with the opening of the tank body. A delaying structure for delaying a motion of the valve body is attached to the support arms. An activator for pulling up the valve body is fitted to the tank body. Delaying the valve closing motion of the valve body allows the stored water within the tank body to be used as flush water for the toilet bowl, until the stored water drops near the bottom.