A toilet flush water supply device for supplying flush water to a flush water tank of a flush toilet is disclosed. The toilet flush water supply device comprises a water supply pipe connected to an external water supply source, a water supply valve for switching between spouting and stopping of flush water supplied from the water supply pipe into the flush water tank, a float for switching between the spouting and stopping of the above water supply valve by a vertical movement thereof linked to fluctuations in the flush water level within the flush water tank, and a float guide disposed on the outer surface of the water supply pipe for guiding the vertical motion of the float.