An electrolyzer comprising an electrolyzer gasket incorporated in the electrolyzer, comprising: an anode; a cathode; and a sheet-like separator that isolates the anode and the cathode, the electrolyzer gasket being for holding the separator, wherein the electrolyzer has: a picture-frame-shaped anode metal frame having an opening for forming an anode chamber in a central part; and a cathode metal frame having the same shape as the anode metal frame and having an opening for forming a cathode chamber in a central part, the anode is attached at the opening of the anode metal frame, the cathode is attached at the opening of the cathode metal frame, and the electrolyzer gasket is sandwiched in a tight state between the anode metal frame and the cathode metal frame, the electrolyzer gasket comprises a single picture-frame-shaped thin-plate-like frame having approximately the same shape as the anode metal frame and the cathode metal frame, the picture-frame-shaped thin-plate-like frame has: a first surface being in close contact with the anode metal frame; and a second surface being in close contact with the cathode metal frame, and has a structure obtained by forming, on any one of the first surface and the second surface, a notch having a difference in level of approximately the same thickness as the thickness of the separator, the notch formed by thinly scraping off, in a uniform thickness, a region including an edge on the anode or the cathode of the thin-plate-like frame, and an edge part of the separator is accommodated in a gap formed between the notch and the anode metal frame or the cathode metal frame, and the separator is held in a state of being nearer to the surface of the anode or the surface of the cathode. the electrolyzer has: a picture-frame-shaped anode metal frame having an opening for forming an anode chamber in a central part; and a cathode metal frame having the same shape as the anode metal frame and having an opening for forming a cathode chamber in a central part, the anode is attached at the opening of the anode metal frame, the cathode is attached at the opening of the cathode metal frame, and the electrolyzer gasket is sandwiched in a tight state between the anode metal frame and the cathode metal frame, the electrolyzer gasket comprises a single picture-frame-shaped thin-plate-like frame having approximately the same shape as the anode metal frame and the cathode metal frame, the picture-frame-shaped thin-plate-like frame has: a first surface being in close contact with the anode metal frame; and a second surface being in close contact with the cathode metal frame, and has a structure obtained by forming, on any one of the first surface and the second surface, a notch having a difference in level of approximately the same thickness as the thickness of the separator, the notch formed by thinly scraping off, in a uniform thickness, a region including an edge on the anode or the cathode of the thin-plate-like frame, and an edge part of the separator is accommodated in a gap formed between the notch and the anode metal frame or the cathode metal frame, and the separator is held in a state of being nearer to the surface of the anode or the surface of the cathode.