A method for manufacturing a spark plug comprising: - a center electrode extending in an axial direction, - an insulator having an axial bore extending there through in the axial direction, and holding the center electrode in an axially forward end portion of the axial bore, and - a tubular metallic shell circumferentially surrounding and holding the insulator, and having a flange-like collar portion protruding radially outward from its outer circumference, - the method comprising - an assembling step of assembling the insulator into the metallic shell through insertion of the insulator into the metallic shell from an axially rear end opening portion of the metallic shell, - whereas the metallic shell is supported by a pedestal, and - the assembling step including a displacement restricting step of restricting a relative positional displacement between the metallic shell and the insulator in a radial direction intersecting with the axial direction so as to reduce eccentricity between an axis of the metallic shell and an axis of the insulator to a predetermined value or less while allowing a relative positional displacement between the metallic shell and the insulator in the axial direction, the method being characterized in that - in the displacement restricting step, after the metallic shell is pressed against the pedestal, the insulator is pressed against the metallic shell while the metallic shell is held in the state of being pressed against the pedestal, and - in the assembling step, the metallic shell is pressed against the pedestal through a seat surface of the collar portion, and a load per unit area of 0.5 N/m 2 or more is imposed on the seat surface of the collar portion, and - in the assembling step, whether or not the insulator is cracked in the assembling step is judged by detecting acoustic emission from the insulator, - whereas the acoustic emission is detected by a sensor which is installed at such a position as to be able to detect the acoustic emission conducted through the pedestal. - a center electrode extending in an axial direction, - an insulator having an axial bore extending there through in the axial direction, and holding the center electrode in an axially forward end portion of the axial bore, and - a tubular metallic shell circumferentially surrounding and holding the insulator, and having a flange-like collar portion protruding radially outward from its outer circumference, - the method comprising - an assembling step of assembling the insulator into the metallic shell through insertion of the insulator into the metallic shell from an axially rear end opening portion of the metallic shell, - whereas the metallic shell is supported by a pedestal, and - the assembling step including a displacement restricting step of restricting a relative positional displacement between the metallic shell and the insulator in a radial direction intersecting with the axial direction so as to reduce eccentricity between an axis of the metallic shell and an axis of the insulator to a predetermined value or less while allowing a relative positional displacement between the metallic shell and the insulator in the axial direction, - in the displacement restricting step, after the metallic shell is pressed against the pedestal, the insulator is pressed against the metallic shell while the metallic shell is held in the state of being pressed against the pedestal, and - in the assembling step, the metallic shell is pressed against the pedestal through a seat surface of the collar portion, and a load per unit area of 0.5 N/m 2 or more is imposed on the seat surface of the collar portion, and - in the assembling step, whether or not the insulator is cracked in the assembling step is judged by detecting acoustic emission from the insulator, - whereas the acoustic emission is detected by a sensor which is installed at such a position as to be able to detect the acoustic emission conducted through the pedestal.