Abstract: Disclosed is an auxiliary water tank device for a toilet stool, to receive and store a small amount of wash water and to supply the stored wash water into a toilet bowl. The auxiliary water tank device includes a housing (10) mounted in a water tank (702) on a bottom surface thereof and having a water supply port (11) and a water discharge port (12), a water supply unit (20) to supply wash water into the housing through the water supply port and to close the water supply port if the housing is filled with the wash water by a predetermined level, a water discharge unit (30) to discharge the wash water filled in the housing through the water discharge port and to close the water discharge port if the wash water is completely discharged, and a control unit (40) to control opening/closing operations of the water discharge unit and an excrement discharge pipe opener (710).