The present invention is a target holding tool in which a guide part has two guide surfaces for holding two side surfaces therebetween which are a surface extending in the extension direction of an end surface of a measurement object and a surface extending in the direction orthogonal to the end surface, target contacting parts for contacting the end surfaces of the measurement object are disposed between the guide surfaces, one of the guide surfaces is provided so as to be able to move so as to approach or separate from the other guide surface fixed to a support part, a connecting part is disposed between the guide surfaces of the guide part on only the other guide surface fixed to the support part, a magnet has flat surfaces running along the side surfaces which are the surface extending in the extension direction of the end surface of the measurement object and the surface extending in the direction orthogonal to the end surface, and is capable of magnetic adsorption to the side surfaces of the measurement object, and the guide part has a space equal to or greater than the thickness of the measurement object on the side-surface sides on the reverse sides of the side surfaces from the flat surfaces of the magnet in a state in which the magnet is magnetically adsorbed to the side surfaces of the measurement object, the magnet being able to move along the side surfaces of the measurement object in a state in which the magnet is magnetically adsorbed to the side surfaces of the measurement object.