Abstract: A method for manufacturing a spark plug that includes preparing an assembly fitted with a cylindrical insulator, and a main metal fitting disposed on an outer periphery of the insulator; photographing the assembly by using a camera disposed on the axis while turning on a first lighting equipment disposed; photographing the assembly by using the camera while turning on a second lighting equipment; specifying a position of an outer peripheral surface of the insulator by using one of photographed images; specifying a position of an inner peripheral surface of the main metal fitting by using the other of the photographed images; and specifying a size of a clearance between the insulator and the main metal fitting by using the position of the outer peripheral surface of the insulator and the position of the inner peripheral surface of the main metal fitting.