To provide a covering device of a sliding door rail which covers the sliding door rail in a state where a sliding door is open, gives no pain even in the case of working by laying an arm or an elbow thereon for a long time in a draft chamber in particular and can improve handleability of a frontage part.

The sliding door rail 3 is provided at the frontage part 1 in a stage falling shape toward a bottom surface part 2 of its back side, sliding doors 4 are provided on the sliding door rail with freely opening/closing, a rail cover 5 is externally fitted on the sliding door rail between inside surfaces 6 of the frontage part facing the sliding door in a state where the sliding door is open and further an upper face of the rail cover is set so as to form about one surface with the bottom surface part.