To eliminate interference with a sash or the like, and to allow a rail for remodeling to be permanently mounted to an existing frame.

The rail device for remodeling 2 is provided for fixing the rail for remodeling 7 on an upper existing frame 5 out of an entire body of the existing frame arranged in a skeleton beforehand along the longitudinal direction of the upper existing frame 5 by using a retaining fitting 1. Specifically a mounting portion 71 of the rail for remodeling 7 is pressed to the rail for remodeling 7 in the direction of the upper existing frame 5 along a cross sectional direction by a pressing portion 1a of the retaining fitting 1, and the edge 71a of the mounting portion 71 of the rail for remodeling 7 mounted to the upper existing frame 5 is supported by a support portion 15 of the retaining fitting 1.