Photoelectrochemical reaction device

A photoelectrochemical reaction device of an embodiment includes: a first stack including a first electrode, a second electrode, and a photovoltaic layer provided therebetween; a second stack including a third electrode electrically connected to the first electrode, a fourth electrode electrically connected to the second electrode, and an ion migration layer provided therebetween; and an electrolytic solution tank storing a first electrolytic solution in which the third electrode is immersed and a second electrolytic solution in which the fourth electrode is immersed. One of the third and fourth electrodes causes an oxidation reaction, and the other of the third and fourth electrodes causes a reduction reaction. The third and fourth electrodes have ion permeability. An area of the second stack is larger than that of the first stack.