NOVELTY - The apparatus (100) has a water supply path (3) that supplies water to a water electrolysis apparatus (1) from a feed water tank (2). The water circulates in a circulation path between electrolysis apparatus and tank through a return path (4) and a water supply path provided at a cathode side by returning water discharged from electrolysis apparatus through return path with decomposition generated gas to tank. A water replenishment apparatus (6) implements replenishment of water based on electric charge consumed for the pressure of the gas, temperature, and electrolysis.    USE - Apparatus for generating hydrogen and oxygen.    ADVANTAGE - The water lost per unit time from a circulation path by electrolyzing water is provided with the water entrained together by forming water vapor when the decomposition generated gas other than the water which is electrolyzed and consumed by hydrogen gas and oxygen gas is discharged from the circulation path so that the replenishment of water is implemented based on the electric charge consumed for electrolysis of water. Thus, the water used as hydrogen gas or oxygen gas is replenished effectively based on the electric charge. Hence, the volume of decomposition generated gas discharged by the pressure and electric charge of decomposition generated gas is grasped easily and the amount of saturated vapor is calculated with the temperature of decomposition generated gas while estimating the water level of feed water tank without measuring directly with a water gauge at stabilized gas-making condition.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for generating hydrogen and oxygen gases.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of the apparatus for generating hydrogen and oxygen.    Water electrolysis apparatus (1)    Feed water tank (2)    Water supply path (3)    Return path (4)    Water replenishment apparatus (6)    Apparatus for generating hydrogen and oxygen (100)