The invention relates to a method for reconditioning a used grinding roller of a material bed roller mill having profile-members which are embedded, with a portion of the length thereof, in holes of the roller body and, with the remainder of the length thereof, project above the surface of the roller body. The housing is configured to thread into a spark-plug hole in an engine. The substrates are positioned in such a manner that the growth faces of the substrates form a wedge. Therefore, even in a case wherein the air bag door portion and the main body portion of the trim member for the automotive vehicle are formed of the same resin, a quality of an outer appearance is not deteriorated, and a break force of a break portion of the air bag door portions is reduced to a desired value.