Abstract: A toilet bowl flushing system according to the invention comprises a toilet bowl flushing device being installable in a low tank having a drain valve and capable of performing an operation of opening the drain valve; and a control unit that stores control information on a plurality of flushing modes, the control unit being capable of programming one of the plurality of flushing modes and supplying the toilet bowl flushing device with a control signal based on the programmed flushing mode. Such a toilet bowl flushing system can selectively use a plurality of flushing modes as appropriate to be adapted to various types of low tanks already on the market. The setting of the flushing mode, as well as the attaching angle of the toilet bowl flushing device, the distance from the output axle to the ball chain lever, the operation angle range of the ball chain lever, and the like can be appropriately varied. As a result, users can benefit from automatic flushing without replacing the low tank.