Patent ID: 8628367

Claim:
A method for measuring a deviation of a member from a joint target position in a spark plug, comprising the steps of: joining a member to a measuring object having an end face at a position other than an axis of the measuring object on the end face; engaging the measuring object with 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 formed; detecting at least one predetermined measuring point 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 measuring a deviation of the member from the joint target position based on an angle between a straight line that connects the axis of the measuring object to the measuring point 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, wherein the deviation is confirmed in numerical terms by analyzing and calculating data obtained from the projection image.