An air bag cover is held by the bag holder in such a manner that the respective side walls of the cover are secured to their corresponding securing piece portions of the vertical walls of the bag holder. The front and rear side walls of the cover respectively include securing grooves to which the front and rear securing piece portions of the bag holder are to be secured. The opening width dimension of the securing groove is set a little smaller than the height dimension of its corresponding securing piece portion, while the opening width dimension of the securing groove is set a little larger than the height dimension of its corresponding securing piece portion. The right and left side walls respectively include securing projecting strips to which the right and left securing piece portions of the bag holder can be secured.