An electrochemical reduction device for electrochemically hydrogenating a benzene ring of an aromatic compound in an organic liquid, wherein a concentration of the aromatic compound decreases over time in response to which a maximum current value I max is decreased, said device comprising: an electrode unit including an electrolyte membrane having ionic conductivity, a reduction electrode that is provided on one side of the electrolyte membrane and includes an reduction catalyst for hydrogenating alkylbenzene containing at least one aromatic ring and with 1 to 4 hydrogen atoms of the aromatic ring being substituted by an alkyl group having 1 to 6 carbon atoms, and an oxygen evolving electrode that is provided on the other side of the electrolyte membrane; a power control unit for applying a voltage V a to one electrode of the reduction electrode and the oxygen evolving electrode thereby producing an electric difference with respect to other electrode thereof; an organic material storage tank that stores the alkylbenzene in an organic liquid and a circulation pathway that connects the organic material storage tank and the reduction electrode of the electrode unit, the organic liquid passing through the circulation pathway; the device further being characterized by: a concentration measurement unit for measuring a concentration of the alkylbenzene supplied to the reduction electrode; and a current detection unit for detecting a current value I flowing through the reduction electrode and the oxygen evolving electrode; a control unit for controlling the power control unit such that the current value I satisfies a relation of I ≤ I max with respect to a maximum current value I max defined according to the concentration C of the alkylbenzene being measured by the concentration measurement unit; wherein the maximum current value I max is defined such that the Faraday efficiency becomes a predetermined value or more, and wherein the maximum current value I max is set to be low as the concentration C of the alkylbenzene is low. an electrode unit including an electrolyte membrane having ionic conductivity, a reduction electrode that is provided on one side of the electrolyte membrane and includes an reduction catalyst for hydrogenating alkylbenzene containing at least one aromatic ring and with 1 to 4 hydrogen atoms of the aromatic ring being substituted by an alkyl group having 1 to 6 carbon atoms, and an oxygen evolving electrode that is provided on the other side of the electrolyte membrane; a power control unit for applying a voltage V a to one electrode of the reduction electrode and the oxygen evolving electrode thereby producing an electric difference with respect to other electrode thereof; an organic material storage tank that stores the alkylbenzene in an organic liquid and a circulation pathway that connects the organic material storage tank and the reduction electrode of the electrode unit, the organic liquid passing through the circulation pathway; the device further being characterized by: a concentration measurement unit for measuring a concentration of the alkylbenzene supplied to the reduction electrode; and a current detection unit for detecting a current value I flowing through the reduction electrode and the oxygen evolving electrode; a concentration measurement unit for measuring a concentration of the alkylbenzene supplied to the reduction electrode; and a current detection unit for detecting a current value I flowing through the reduction electrode and the oxygen evolving electrode; a control unit for controlling the power control unit such that the current value I satisfies a relation of I ≤ I max with respect to a maximum current value I max defined according to the concentration C of the alkylbenzene being measured by the concentration measurement unit; wherein the maximum current value I max is defined such that the Faraday efficiency becomes a predetermined value or more, and wherein the maximum current value I max is set to be low as the concentration C of the alkylbenzene is low.