A water electrolysis and electricity generating system is equipped with a second supply flow path, a second lead-out flow path, a second gas-liquid separator, a hydrogen exhaust gas circulation flow path, and a storage flow path. In the second lead-out flow path, product hydrogen gas and hydrogen exhaust gas are led out from a cell member. The second gas-liquid separator separates into a gas and a liquid the product hydrogen gas and the hydrogen exhaust gas which have been led out from the second lead-out flow path. The second lead-out flow path and the second gas-liquid separator are shared in common by a water electrolysis mode and an electricity generating mode.