Provided are a water decomposition apparatus and a water decomposition method that can maintain high gas generation efficiency even in an early stage of light irradiation and even in a case where time has elapsed and that can recover the gas generation amount of hydrogen gas or the like, can generate hydrogen gas or the like stably for a long time on an average, and can increase the integrated amount of generation of hydrogen for a long time, even in a case where time has elapsed and the gas generation amount of hydrogen gas or the like has decreased.