A hydrogen generation device (100, 200, 300, 400, 500, 600, 700) comprising: a transparent substrate; a photocatalytic electrode formed of a transparent conductive layer disposed on the transparent substrate and a photocatalytic layer disposed on the transparent conductive layer; a counter electrode connected electrically to the transparent conductive layer; a water-containing electrolyte solution layer provided between the photocatalytic electrode and the counter electrode; a separator that separates the electrolyte solution layer into a first electrolyte solution layer in contact with the photocatalytic electrode and a second electrolyte solution layer in contact with the counter electrode; a first gas outlet, connected to the first electrolyte solution layer, for discharging oxygen gas or hydrogen gas generated in the first electrolyte solution layer; and a second gas outlet, connected to the second electrolyte solution layer, for discharging hydrogen gas or oxygen gas generated in the second electrolyte solution layer, wherein the photocatalytic electrode and the counter electrode are arranged so that a surface of the photocatalytic layer and a surface of the counter electrode face each other, the separator allows an electrolyte in the electrolyte solution layer to pass therethrough and prevents hydrogen gas and oxygen gas in the electrolyte solution layer from passing therethrough, and wherein the photocatalytic electrode is configured to be irradiated with light from a side of the transparent substrate with respect to the transparent conductive layer, the hydrogen generation device (100, 200, 300, 400, 500, 600, 700) further comprising a fixing and supporting member that fixes the separator in place and supports the separator, wherein the fixing and supporting member fixes and supports the separator so that the separator is disposed with predetermined distances from the surface of the photocatalytic layer and the surface of the counter electrode, respectively, and in parallel with the surfaces, wherein the surface of the photocatalytic layer, made of a semiconductor material acting as a photocatalyst, on a side of the first electrolyte solution layer is in contact only with the first electrolyte solution layer, the fixing and supporting member, and a sealing material of the first electrolyte solution layer. a transparent substrate; a photocatalytic electrode formed of a transparent conductive layer disposed on the transparent substrate and a photocatalytic layer disposed on the transparent conductive layer; a counter electrode connected electrically to the transparent conductive layer; a water-containing electrolyte solution layer provided between the photocatalytic electrode and the counter electrode; a separator that separates the electrolyte solution layer into a first electrolyte solution layer in contact with the photocatalytic electrode and a second electrolyte solution layer in contact with the counter electrode; a first gas outlet, connected to the first electrolyte solution layer, for discharging oxygen gas or hydrogen gas generated in the first electrolyte solution layer; and a second gas outlet, connected to the second electrolyte solution layer, for discharging hydrogen gas or oxygen gas generated in the second electrolyte solution layer, wherein the photocatalytic electrode and the counter electrode are arranged so that a surface of the photocatalytic layer and a surface of the counter electrode face each other, the separator allows an electrolyte in the electrolyte solution layer to pass therethrough and prevents hydrogen gas and oxygen gas in the electrolyte solution layer from passing therethrough, and wherein the photocatalytic electrode is configured to be irradiated with light from a side of the transparent substrate with respect to the transparent conductive layer, the hydrogen generation device (100, 200, 300, 400, 500, 600, 700) further comprising a fixing and supporting member that fixes the separator in place and supports the separator, wherein