An electrochemical reaction device, comprises: an anode to oxidize a first substance; a first flow path facing on the anode and through which a liquid containing the first substance flows; a cathode to reduce a second substance; a second flow path facing on the cathode and through which a gas containing the second substance flows; a porous separator provided between the anode and the cathode; and a power supply connected to the anode and the cathode. A thickness of the porous separator is 1 μm or more and 500 μm or less. An average fine pore size of the porous separator is larger than 0.008 μm and smaller than 0.45 μm. A porosity of the porous separator is higher than 0.5.