A photochemical reaction system, comprising: a CO 2 production unit for producing CO 2; a CO 2 absorption unit for absorbing CO 2 produced by the CO 2 production unit; and as a CO 2 reduction unit, a photochemical reaction device having a photoelectrochemical cell that has a laminated body comprising: an oxidation catalyst layer for producing O 2 and H + by oxidizing H 2 O; a reduction catalyst layer for producing carbon compounds by reducing CO 2 absorbed by the CO 2 absorption unit; and a multi-junction photovoltaic cell formed between the oxidation catalyst layer and the reduction catalyst layer, the multi-junction photovoltaic cell contains a semiconductor layer developing charge separation with light energy; and wherein the photochemical reaction device further has an ion transfer pathway making ions move between the oxidation catalyst layer side and the reduction catalyst layer side, and wherein the oxidation catalyst layer is capable of oxidizing a reductant and the reduction catalyst layer is capable of reducing CO 2 by using an electron generated by oxidizing the reductant, the oxidation catalyst layer is made from manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, nickel-cobalt-oxide, lanthanum-cobalt-oxide, nickel-lanthanum-oxide, strontium-iron-oxide, lead-ruthenium-iridium-oxide, lanthanum-strontium-cobalt-oxide, a ruthenium complex or an iron complex, and the reduction catalyst layer is made from Au, Ag, Cu, Pt, Ni, Zn, C, graphene, CNT (carbon nanotube), fullerene, Ketjen black, Pd, an alloy including at least one of them, a ruthenium complex or a rhenium complex. a CO 2 production unit for producing CO 2; a CO 2 absorption unit for absorbing CO 2 produced by the CO 2 production unit; and as a CO 2 reduction unit, a photochemical reaction device having a photoelectrochemical cell that has a laminated body comprising: an oxidation catalyst layer for producing O 2 and H + by oxidizing H 2 O; a reduction catalyst layer for producing carbon compounds by reducing CO 2 absorbed by the CO 2 absorption unit; and a multi-junction photovoltaic cell formed between the oxidation catalyst layer and the reduction catalyst layer, the multi-junction photovoltaic cell contains a semiconductor layer developing charge separation with light energy; and wherein the photochemical reaction device further has an ion transfer pathway making ions move between the oxidation catalyst layer side and the reduction catalyst layer side, and wherein the oxidation catalyst layer is capable of oxidizing a reductant and the reduction catalyst layer is capable of reducing CO 2 by using an electron generated by oxidizing the reductant, the oxidation catalyst layer is made from manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, nickel-cobalt-oxide, lanthanum-cobalt-oxide, nickel-lanthanum-oxide, strontium-iron-oxide, lead-ruthenium-iridium-oxide, lanthanum-strontium-cobalt-oxide, a ruthenium complex or an iron complex, and the reduction catalyst layer is made from Au, Ag, Cu, Pt, Ni, Zn, C, graphene, CNT (carbon nanotube), fullerene, Ketjen black, Pd, an alloy including at least one of them, a ruthenium complex or a rhenium complex. an oxidation catalyst layer for producing O 2 and H + by oxidizing H 2 O; a reduction catalyst layer for producing carbon compounds by reducing CO 2 absorbed by the CO 2 absorption unit; and a multi-junction photovoltaic cell formed between the oxidation catalyst layer and the reduction catalyst layer, the multi-junction photovoltaic cell contains a semiconductor layer developing charge separation with light energy; and wherein the photochemical reaction device further has an ion transfer pathway making ions move between the oxidation catalyst layer side and the reduction catalyst layer side, and wherein the oxidation catalyst layer is capable of oxidizing a reductant and the reduction catalyst layer is capable of reducing CO 2 by using an electron generated by oxidizing the reductant, the oxidation catalyst layer is made from manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, nickel-cobalt-oxide, lanthanum-cobalt-oxide, nickel-lanthanum-oxide, strontium-iron-oxide, lead-ruthenium-iridium-oxide, lanthanum-strontium-cobalt-oxide, a ruthenium complex or an iron complex, and the reduction catalyst layer is made from Au, Ag, Cu, Pt, Ni, Zn, C, graphene, CNT (carbon nanotube), fullerene, Ketjen black, Pd, an alloy including at least one of them, a ruthenium complex or a rhenium complex.