To provide an innovative retrofit method for overcoming difficulties with covering method and dismantling method as general methods of retrofitting a fitting frame or the like, including a decrease in the size of openings, pollution resulting from a variety of construction work for dismantling, the cost of repairs, an increase in working hours, and the like, so as to significantly reduce the pollution resulting from the variety of construction work, the time required, the cost of repairs, etc.

The required minimum portions of an existing frame, including its exposed portions other than margins for connections, and padding or the like that may obstruct mounting of a new metallic frame or the like, are cut away and the buried portions of the existing frame are used as anchors for the new metallic frame. Also, the margins for connections of the existing frame are lap welded to the margins for connections of the new frame having an opening of the same size, and the frames are locked to each other with small screws or the like. The new frame is installed by assembling two split parts, an outer frame and an inner frame, and connecting and locking the split frames together using an engagement device. Padding such as dry mortar is packed into gaps in the new frame for retrofitting.