An electrochemical reactor 70 is provided with a proton conductive solid electrolyte layer 75; an anode layer 76 arranged on the surface of the solid electrolyte layer and able to hold water molecules; a cathode layer 77 arranged on the surface of the solid electrolyte layer; and a current control device 73 controlling a current flowing through the anode layer and the cathode layer. The current control device reduces the current flowing through the anode layer and the cathode layer, when the water molecules held in the anode layer become smaller in amount.