To provide a method and an apparatus for producing a sodium hypochlorite solution on-site at a high efficiency and at the initial cost and suppressing operating cost without any problems about impurities derived from raw material water and raw salt. A method of producing a sodium hypochlorite solution on-site in the vicinity of a physical plant where a sodium hypochlorite solution is used. In production of a sodium hypochlorite solution by feeding secondary salt water, in an electrolyzer comparted into an anode chamber and a cathode chamber with an ion-exchange membrane, to the anode chamber and allowing components in the anode chamber and the cathode chamber after electrolysis to react in a reaction tank, there are included respective steps of treating raw material water A with a cation-exchange resin B to generate purified water 5, dissolving raw salt D in the purified water to generate primary salt water E, performing an examination for confirming the presence or absence of a precipitate or suspended solid in the primary salt water, and directly performing chelating in the case of no precipitate or suspended solid contained in the primary salt water, or performing chelating after addition of an acid component for dissolution of the precipitate or suspended solid in the case of the precipitate or suspended solid contained therein, to generate secondary salt water 4.