A water electrolysis device for generating hydrogen gas includes a case, a power supplying unit, and an ion-exchange membrane electrolyzer. The case has a containing space including a bottom space and a top space. The bottom space is larger than the top space. The power supplying unit is configured in the bottom space for supplying power to operate the water electrolysis device. The ion-exchange membrane electrolyzer includes an ion-exchange membrane and a cathode, and the cathode generates hydrogen gas during which the ion-exchange membrane electrolyzer electrolyzes water.