An alkaline water electrolyzer comprising: an anode chamber frame defining an anode chamber; a cathode chamber frame defining a cathode chamber; an anode accommodated in the anode chamber; a cathode accommodated in the cathode chamber; a porous diaphragm disposed between the anode and cathode chamber frames (11, 17) and partitioning the anode and cathode chambers the electrolyzer being characterised by the following: an anode gasket disposed on an anode chamber frame surface where the porous diaphragm is disposed; and a cathode gasket disposed on a cathode chamber frame surface where the porous diaphragm is disposed, wherein when the anode chamber frame and the cathode chamber frame (11, 17) are fastened to each other, the porous diaphragm is held between the anode chamber frame and the cathode chamber frame (11, 17) via the anode gasket and the cathode gasket and the anode gasket and the cathode gasket (15, 21) are in contact with each other around a peripheral edge of the porous diaphragm by compressing the anode gasket and the cathode gasket; and wherein the anode gasket and the cathode gasket have a same shape, and the alkaline water electrolyzer satisfies Formulae to below: A ≥ 3 mm W − A ≥ 3 mm and D − D ′ × 2 − T ≥ 0.01 mm where W represents widths of the anode gasket and the cathode gasket, D represents thicknesses of the anode gasket and the cathode gasket before the compression, D' represents thicknesses of inner peripheral portions of the anode gasket and the cathode gasket after the compression, A represents a contact width between the porous diaphragm and the anode gasket and the cathode gasket, and T represents a thickness of the porous diaphragm during the compression. an anode chamber frame defining an anode chamber; a cathode chamber frame defining a cathode chamber; an anode accommodated in the anode chamber; a cathode accommodated in the cathode chamber; a porous diaphragm disposed between the anode and cathode chamber frames (11, 17) and partitioning the anode and cathode chambers the electrolyzer being characterised by the following: an anode gasket disposed on an anode chamber frame surface where the porous diaphragm is disposed; and a cathode gasket disposed on a cathode chamber frame surface where the porous diaphragm is disposed, wherein when the anode chamber frame and the cathode chamber frame (11, 17) are fastened to each other, the porous diaphragm is held between the anode chamber frame and the cathode chamber frame (11, 17) via the anode gasket and the cathode gasket and the anode gasket and the cathode gasket (15, 21) are in contact with each other around a peripheral edge of the porous diaphragm by compressing the anode gasket and the cathode gasket; and wherein the anode gasket and the cathode gasket have a same shape, and the alkaline water electrolyzer satisfies Formulae to below: A ≥ 3 mm W − A ≥ 3 mm and D − D ′ × 2 − T ≥ 0.01 mm where W represents widths of the anode gasket and the cathode gasket, D represents thicknesses of the anode gasket and the cathode gasket before the compression, D' represents thicknesses of inner peripheral portions of the anode gasket and the cathode gasket after the compression, A represents a contact width between the porous diaphragm and the anode gasket and the cathode gasket, and T represents a thickness of the porous diaphragm during the compression. an anode gasket disposed on an anode chamber frame surface where the porous diaphragm is disposed; and a cathode gasket disposed on a cathode chamber frame surface where the porous diaphragm is disposed, wherein when the anode chamber frame and the cathode chamber frame (11, 17) are fastened to each other, the porous diaphragm is held between the anode chamber frame and the cathode chamber frame (11, 17) via the anode gasket and the cathode gasket and the anode gasket and the cathode gasket (15, 21) are in contact with each other around a peripheral edge of the porous diaphragm by compressing the anode gasket and the cathode gasket; and wherein the anode gasket and the cathode gasket have a same shape, and the alkaline water electrolyzer satisfies Formulae to below: A ≥ 3 mm W − A ≥ 3 mm and D − D ′ × 2 − T ≥ 0.01 mm W represents widths of the anode gasket and the cathode gasket, D represents thicknesses of the anode gasket and the cathode gasket before the compression, D' represents thicknesses of inner peripheral portions of the anode gasket and the cathode gasket after the compression, A represents a contact width between the porous diaphragm and the anode gasket and the cathode gasket, and T represents a thickness of the porous diaphragm during the compression.