A starting method includes determining whether a depressurizing current was supplied to a water electrolyzer while at least a cathode side of the water electrolyzer was depressurized in an immediately previous stop of a water electrolysis system after electrolyzing water. A first current is supplied to the water electrolyzer at a first supply rate to start the water electrolysis system in a case where it is determined that the depressurizing current was supplied to the water electrolyzer in the immediately previous stop. A second current is supplied to the water electrolyzer at a second supply rate lower than the first supply rate to start the water electrolysis system in a case where it is determined that the depressurizing current was not supplied to the water electrolyzer in the immediately previous stop.