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 . <EOS>