To prevent deposition of scale and water scale on an inner peripheral wall of a toilet stool without polluting flush water in the toilet stool and in an environment-friendly manner.

There is provided a flush toilet stool cleaning method, according to which a cleaning agent is stored in a tank for flushing the toilet stool, and the inner wall surface of the toilet stool and flush water per se are cleaned by the cleaning agent dissolved in the water at the time of flushing to feed water into the toilet stool. The method is characterized in that the cleaning agent is made of a water-soluble amorphous body in the form of a bulk material, a granular material, and a powdery material.