A zeta-potential measurement jig, to be used for an electrophoretic mobility measuring device, comprising: a frame body including: a first holding wall and a second holding wall, which are arranged in such a manner as to be opposed to each other, and each have an opening formed at a corresponding position, the openings configured to pass light to irradiate a sample through one of the openings and to pass light scattered by the sample through another one of the openings; and a bottom wall configured to couple a lower end of the first holding wall and a lower end of the second holding wall to each other; an anode plate and a cathode plate which are provided on the bottom wall; a cell; an intermediate block, which is configured to form part of a holding space for holding the sample between the first holding wall and the second holding wall, and is arranged above or below the cell in such a manner as to be adjacent to the cell, the cell being arranged between the openings at substantially the same level as that of the openings, and a cell retainer, which is arranged above the intermediate block between the first holding wall and the second holding wall, and is configured to press the cell and the intermediate block toward the bottom wall, wherein at least one of the first holding wall or the second holding wall has one of a first groove or a first protrusion to be elastically fitted into the first groove, which is configured to support the intermediate block on a lateral side, and wherein the intermediate block has another one of the first groove or the first protrusion. a frame body including: a first holding wall and a second holding wall, which are arranged in such a manner as to be opposed to each other, and each have an opening formed at a corresponding position, the openings configured to pass light to irradiate a sample through one of the openings and to pass light scattered by the sample through another one of the openings; and a bottom wall configured to couple a lower end of the first holding wall and a lower end of the second holding wall to each other; a first holding wall and a second holding wall, which are arranged in such a manner as to be opposed to each other, and each have an opening formed at a corresponding position, the openings configured to pass light to irradiate a sample through one of the openings and to pass light scattered by the sample through another one of the openings; and a bottom wall configured to couple a lower end of the first holding wall and a lower end of the second holding wall to each other; an anode plate and a cathode plate which are provided on the bottom wall; a cell; an intermediate block, which is configured to form part of a holding space for holding the sample between the first holding wall and the second holding wall, and is arranged above or below the cell in such a manner as to be adjacent to the cell, the cell being arranged between the openings at substantially the same level as that of the openings, and a cell retainer, which is arranged above the intermediate block between the first holding wall and the second holding wall, and is configured to press the cell and the intermediate block toward the bottom wall, wherein at least one of the first holding wall or the second holding wall has one of a first groove or a first protrusion to be elastically fitted into the first groove, which is configured to support the intermediate block on a lateral side, and wherein the intermediate block has another one of the first groove or the first protrusion.