An additive-storing tank assembly includes: a tank body having an additive-storing space defined in the tank body, in which an additive is stored; a plunger-holding tube provided at one side of the tank body in order to connect the additive-storing space with a water supply pipe for supplying water to a dispenser of a refrigerator; and an opening/closing means for selectively connecting the additive-storing space with the water supply pipe so that the additive stored in the additive-storing space is dissolved in the water flowing through the water supply pipe.