To provide an auxiliary decorative member and a method and structure for mounting the auxiliary decorative member capable of making the edges of decorative frames beautiful in appearance even if two decorative frames of different lengths are mounted perpendicularly to each other to correspond to a gap between each decorative frame and a wall face.

The auxiliary decorative member 1 is divided into two auxiliary decorative member pieces 1a and 101 even if two decorative frames of different lengths are mounted perpendicular to each other. By appropriately separating laminate pieces 1f and 101f which constitute the auxiliary decorative member 1, the thickness of each of the auxiliary decorative members pieces 1a and 101 is adjusted. The auxiliary decorative member pieces are provided perpendicular to each other such that on the back of one of the member pieces 1a, the dividing surface of the other member piece 10a abuts, so that the portion between the edges 4a and 104a of the decorative frames 4 and 104 is easily and surely made beautiful in appearance.