An electrochemical reduction device comprising: a liquid supply device that supplies an aromatic hydrocarbon compound or a nitrogen-containing heterocyclic aromatic compound, selected from the group consisting of benzene, naphthalene, anthracene, diphenylethane, pyridine, pyrimidine, pyrazine, quinoline, isoquinoline, N-alkylpyrrole, N-alkylindole, N-alkyldibenzopyrrole, wherein 1 to 4 hydrogen atoms of the aromatic ring of the aromatic hydrocarbon compound or nitrogen-containing heterocyclic aromatic compound being optionally substituted by linear or branched alkyl group (s) having 1 to 6 carbon atoms, from an organic material storage tank to a reduction electrode of an electrode unit in a liquid state at a reaction temperature; the electrode unit including an electrolyte membrane having ionic conductivity, the reduction electrode that is provided on one side of the electrolyte membrane and that contains a reduction catalyst for hydrogenating a benzene ring of the aromatic hydrocarbon compound or the nitrogen-containing heterocyclic aromatic compound, and an oxygen evolving electrode that is provided on the other side of the electrolyte membrane; a power control unit that applies a voltage Va between the reduction electrode and the oxygen evolving electrode so that the reduction electrode has a basic potential and the oxygen evolving electrode has a noble potential; hydrogen gas generation rate measurement means for measuring a generation rate F1 per unit time of a hydrogen gas generated by an electrolytic reaction of water which competes with a benzene ring hydrogenation reaction of the aromatic hydrocarbon compound or the nitrogen-containing heterocyclic aromatic compound; and a control unit that controls the power control unit so as to gradually increase the voltage Va within a range that satisfies a relationship of F1 ≤ F0 and V CA > V HER - (acceptable potential difference), where the potential at a reversible hydrogen electrode, the potential of the reduction electrode and the acceptable upper limit of the hydrogen gas generation rate are expressed as V HER, V CA and F0, respectively, and the acceptable potential difference is defined as a potential difference that defines an upper limit of a potential difference between V CA and V HER, wherein the acceptable potential difference is 20 mV, a reference electrode that is arranged to be in contact with the electrolyte membrane and to be electrically isolated from the reduction electrode and the oxygen evolving electrode and that is held at a reference electrode potential V Ref; and a voltage detection unit that detects a potential difference ΔV CA between the reference electrode and the reduction electrode, wherein the control unit acquires the potential V CA of the reduction electrode based on the potential difference ΔV CA and the reference electrode potential V REF. a liquid supply device that supplies an aromatic hydrocarbon compound or a nitrogen-containing heterocyclic aromatic compound, selected from the group consisting of benzene, naphthalene, anthracene, diphenylethane, pyridine, pyrimidine, pyrazine, quinoline, isoquinoline, N-alkylpyrrole, N-alkylindole, N-alkyldibenzopyrrole, wherein 1 to 4 hydrogen atoms of the aromatic ring of the aromatic hydrocarbon compound or nitrogen-containing heterocyclic aromatic compound being optionally substituted by linear or branched alkyl group (s) having 1 to 6 carbon atoms, from an organic material storage tank to a reduction electrode of an electrode unit in a liquid state at a reaction temperature; the electrode unit including an electrolyte membrane having ionic conductivity, the reduction electrode that is provided on one side of the electrolyte membrane and that contains a reduction catalyst for hydrogenating a benzene ring of the aromatic hydrocarbon compound or the nitrogen-containing heterocyclic aromatic compound, and an oxygen evolving electrode that is provided on the other side of the electrolyte membrane; a power control unit that applies a voltage Va between the reduction electrode and the oxygen evolving electrode so that the reduction electrode has a basic potential and the oxygen evolving electrode has a noble potential; hydrogen gas generation rate measurement means for measuring a generation rate F1 per unit time of a hydrogen gas generated by an electrolytic reaction of water which competes with a benzene ring hydrogenation reaction of the aromatic hydrocarbon compound or the nitrogen-containing heterocyclic aromatic compound; and a control unit that controls the power control unit so as to gradually increase the voltage Va within a range that satisfies a relationship of F1 ≤ F0 and V CA > V HER - (acceptable potential difference), where the potential at a reversible hydrogen electrode, the potential of the reduction electrode and the acceptable upper limit of the hydrogen gas generation rate are expressed as V HER, V CA and F0, respectively, and the acceptable potential difference is defined as a potential difference that defines an upper limit of a potential difference between V CA and V HER, wherein the acceptable potential difference is 20 mV, a reference electrode that is arranged to be in contact with the electrolyte membrane and to be electrically isolated from the reduction electrode and the oxygen evolving electrode and that is held at a reference electrode potential V Ref; and a voltage detection unit that detects a potential difference ΔV CA between the reference electrode and the reduction electrode, wherein the control unit acquires the potential V CA of the reduction electrode based on the potential difference ΔV CA and the reference electrode potential V REF.