An electrochemical reaction device comprising: a reaction unit comprising a first accommodation part configured to accommodate carbon dioxide, a first liquid or gas supply flow path connected to the first accommodation part for supplying a first electrolytic solution containing carbon dioxide or a gas containing carbon dioxide, a first gas and liquid discharge flow path connected to the first accommodation part, a second accommodation part configured to accommodate a second electrolytic solution containing water, and a diaphragm provided between the first accommodation part and the second accommodation part; a reduction electrode provided in a manner to be in contact with the gas or first electrolytic solution containing the carbon dioxide and configured to reduce the carbon dioxide; an oxidation electrode provided in a manner to be in contact with the second electrolytic solution containing water and configured to oxidize the water; a first product separator which is connected to the first accommodation part through the first gas and liquid discharge flow path; a power supply connected to the reduction electrode and the oxidation electrode; a first pressure regulator which is a variable throttle or a flow rate control valve provided in the first gas and liquid discharge flow path and is configured to regulate a pressure in the first accommodation part; a second pressure regulator configured to regulate a pressure in the second accommodation part; a reaction product detector configured to detect an amount and a kind of a substance produced at the reduction electrode; and a controller configured to control the first pressure regulator second pressure regulator, wherein the controller is configured to control the first pressure regulator so that the potential of the reduction electrode is adjusted to a desired potential, and the amount and the kind of the substance produced at the reduction electrode are adjusted to desired states, based on a detection signal of the reaction product detector, to control the first pressure regulator so that the pressure in the first accommodation part becomes 0.1 MPa or more and 6.4 MPa or less, and to control the first pressure regulator and the second pressure regulator so that a difference between the pressure in the first accommodation part and the pressure in the second accommodation part becomes 0.5 MPa or less. a reaction unit comprising a first accommodation part configured to accommodate carbon dioxide, a first liquid or gas supply flow path connected to the first accommodation part for supplying a first electrolytic solution containing carbon dioxide or a gas containing carbon dioxide, a first gas and liquid discharge flow path connected to the first accommodation part, a second accommodation part configured to accommodate a second electrolytic solution containing water, and a diaphragm provided between the first accommodation part and the second accommodation part; a reduction electrode provided in a manner to be in contact with the gas or first electrolytic solution containing the carbon dioxide and configured to reduce the carbon dioxide; an oxidation electrode provided in a manner to be in contact with the second electrolytic solution containing water and configured to oxidize the water; a first product separator which is connected to the first accommodation part through the first gas and liquid discharge flow path; a power supply connected to the reduction electrode and the oxidation electrode; a first pressure regulator which is a variable throttle or a flow rate control valve provided in the first gas and liquid discharge flow path and is configured to regulate a pressure in the first accommodation part; a second pressure regulator configured to regulate a pressure in the second accommodation part; a reaction product detector configured to detect an amount and a kind of a substance produced at the reduction electrode; and a controller configured to control the first pressure regulator second pressure regulator, wherein the controller is configured to control the first pressure regulator so that the potential of the reduction electrode is adjusted to a desired potential, and the amount and the kind of the substance produced at the reduction electrode are adjusted to desired states, based on a detection signal of the reaction product detector, to control the first pressure regulator so that the pressure in the first accommodation part becomes 0.1 MPa or more and 6.4 MPa or less, and to control the first pressure regulator and the second pressure regulator so that a difference between the pressure in the first accommodation part and the pressure in the second accommodation part becomes 0.5 MPa or less.