A method for measuring a deviation of a member from a joint target position, comprising: a measuring object having an end face and to which a member is joined at a position other than an axis of the measuring object on the end face; a female thread jig having a reference surface in which a reference portion serving as a reference for the joint target position of the member in the measuring object is provided, wherein the measuring object is a metallic shell for a spark plug, and wherein the member is a ground electrode joined to the metallic shell, the method, further comprising: engaging the measuring object with the female thread jig; detecting at least one predetermined measuring point (P1, P2) in a projection image of a front end portion of the member when the front end portion is projected on a flat measurement face including the reference surface while the measuring object is engaged with the female thread jig; detecting the reference portion on the measurement face; and the method being further characterized in: measuring a deviation of the member from the joint target position based on an angle between a straight line (L1, L2) that connects an axis of the measuring object to the measuring point (P1, P2) on the measurement face and a reference straight line that connects the axis of the measuring object to the reference portion on the measurement face. a measuring object having an end face and to which a member is joined at a position other than an axis of the measuring object on the end face; a female thread jig having a reference surface in which a reference portion serving as a reference for the joint target position of the member in the measuring object is provided, wherein the measuring object is a metallic shell for a spark plug, and wherein the member is a ground electrode joined to the metallic shell, the method, further comprising: engaging the measuring object with the female thread jig; detecting at least one predetermined measuring point (P1, P2) in a projection image of a front end portion of the member when the front end portion is projected on a flat measurement face including the reference surface while the measuring object is engaged with the female thread jig; detecting the reference portion on the measurement face; and the method being further characterized in: measuring a deviation of the member from the joint target position based on an angle between a straight line (L1, L2) that connects an axis of the measuring object to the measuring point (P1, P2) on the measurement face and a reference straight line that connects the axis of the measuring object to the reference portion on the measurement face. engaging the measuring object with the female thread jig; detecting at least one predetermined measuring point (P1, P2) in a projection image of a front end portion of the member when the front end portion is projected on a flat measurement face including the reference surface while the measuring object is engaged with the female thread jig; detecting the reference portion on the measurement face; and the method being further characterized in: measuring a deviation of the member from the joint target position based on an angle between a straight line (L1, L2) that connects an axis of the measuring object to the measuring point (P1, P2) on the measurement face and a reference straight line that connects the axis of the measuring object to the reference portion on the measurement face.