Provided is a device for storing and dissolving a chemical to prepare a chemical solution, in which a chemical for disinfection is dissolved and to supply the chemical solution to a ballast water piping. More particularly, the present invention relates to a device for storing and dissolving a chemical for disinfecting ballast water, in which a cylindrical holding case is fixed/installed in the vertical direction on the upper portion of a chemical dissolution tank, in which a solvent (fresh water or seawater) is introduced and stored, a chemical storage barrel having a circular or polygonal shape is inserted/installed in the vertical direction into the holding case, and a plurality of vertical chemical storage chambers are formed in the inner space of the chemical storage barrel by a radial partition plate. Further, a chemical injection opening is formed on the bottom surface of the holding case so as to correspond to one chemical storage chamber, and the chemical storage barrel is rotated in an upright state to perform quantitative injection of the chemical through the chemical dissolution tank in a natural drop type such that each chemical storage chamber sequentially corresponds to the chemical injection opening, thereby more safely managing and using an oxidizer type chemical having toxicity harmful to human bodies. Further, while each chemical storage chamber stores the exact amount of chemical required for disinfecting ballast water once, the corresponding chemical can be injected into the chemical dissolution tank without a remnant, thereby preventing unnecessary waste of the chemical and the solvent and rapidly and accurately adjusting the concentration of the required chemical solution even though a complicated control system for preparing the concentration of the chemical solution is not applied.