NOVELTY - Water electrolysis system (10) comprises: a water electrolysis apparatus (12) comprising electrolyte membrane and current collectors disposed on respective opposite anode and cathode sides of the electrolyte membrane, where the water electrolysis apparatus electrolyzes water to generate oxygen on the anode side and hydrogen on the cathode side; and a casing having an accommodating chamber accommodating the water electrolysis apparatus in it. The casing comprises: a first electric component compartment; and a second electric component compartment.    USE - Used as water electrolysis system.    ADVANTAGE - The system utilizes second electric component compartment that is free of any air blowers (e.g. fan) or has less number of fans that are required to maintain a positive pressure in the system, thus making the system simple and economical and making it possible to accommodate various electronic components in an appropriate environment.    DETAILED DESCRIPTION - Water electrolysis system (10) comprises: a water electrolysis apparatus (12) comprising an electrolyte membrane and current collectors disposed on respective opposite anode and cathode sides of the electrolyte membrane, where the water electrolysis apparatus electrolyzes water to generate oxygen on the anode side and hydrogen on the cathode side; and a casing having an accommodating chamber accommodating the water electrolysis apparatus in it. The casing comprises: a first electric component compartment separate from the accommodating chamber and housing a first electric component in it, where the first electric component compartment has a first fan for introducing external air; and a second electric component compartment separate from the accommodating chamber and housing a second electric component in it, where the second electric component compartment is connected to the first electric component compartment by a pipe. An INDEPENDENT CLAIM is also included for operating the water electrolysis system comprising: (a) keeping the electric component compartment under a positive pressure, while ventilating the electric component compartment; (b) keeping the accommodating chamber under a negative pressure, while ventilating the accommodating chamber after the step (a) is started; and (c) starting to operate the water electrolysis apparatus after the step (b) is started.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view (partly in block form) of the water electrolysis system.    Water electrolysis system (10)    Water electrolysis apparatus (12)    Water circulating apparatus (14)    Gas-liquid separating apparatus (16)    Controller (20)