A method for measuring a deviation from a joint target position of a member to which a measuring object is joined, in particular, measuring the deviation of an electrode position of a spark plug. The deviation of a ground electrode from an optimal igniting position O1 is measured while a metallic shell is engaged with a female thread jig. A distance “ε” between an axis A of the ground electrode and a reference straight line LS is calculated. The distance “ε” is a vertical length from the axis A to the reference straight line LS. An angle “α” between a first straight line L1 and the reference straight line LS is calculated using the distance “ε” and a predetermined distance “β” between the axis A of the ground electrode and an axis O of the metallic shell.