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

A flush water tank device that stores flush water for flushing a flush toilet, the flush water tank device comprising:

• a flush water tank main body that includes a discharge port and is configured to store the flush water;
• a water supply valve that causes the flush water supplied from a water supply source to flow into the flush water tank main body;
• and
• a discharge valve that is provided in the flush water tank main body and is configured to open and close the discharge port to switch between discharging and stopping of the flush water stored in the flush water tank main body to the flush toilet,
• wherein during a flush of the flush toilet, the discharge valve is opened until a water level in the flush water tank main body is lowered from a predetermined full water level to a predetermined dead water level, a lowering speed of the water level in the flush water tank main body changes at a predetermined inflection water level between the full water level and the dead water level,
• wherein the flush water tank main body is formed so that a horizontal cross-sectional area on a side upper than the inflection water level is larger than a horizontal cross-sectional area on a side lower than the inflection water level, thereby the lowering speed of the water level between the inflection water level and the dead water level is faster than the lowering speed of the water level between the full water level and the inflection water level, and
• wherein the water supply valve is configured to cause the flush water supplied from the water supply source to flow into the flush water tank main body during a time period from when the discharge valve is opened until the water level in the flush water tank main body is lowered from the full water level to the inflection water level, and to stop the flush water flowing in from the water supply source or reduce a flow rate of the flush water flowing in the flush water tank main body when the water level in the flush water tank main body becomes lower than the inflection water level.