A salt water supply unit includes a salt water plate that divides the interior of a salt water tank into a salt container and a salt water reservoir, and a salt water well that stands and penetrates the salt water plate. The salt water well accommodates a salt water valve device and a concentration detector. The salt water valve device includes a valve box having a valve hole that allows makeup water or salt water to flow therethrough, a float rod penetrating the valve hole, a valve element coupled to a first end of the float rod, and a water level detecting float coupled to a second end of the float rod. The concentration detector includes a switch that is incorporated in a stem holding a concentration detecting float. The switch outputs different detection signals in accordance with the position of the concentration detecting float.