To provide a fitting structure of a single sliding door easy in construction of the single sliding door, and capable of cleaning a storage space of the single sliding door composed of a double wall.

A wall adjacent to an opening part 50 of the single sliding door 20 is a double wall structure composed of an inner wall 30 and an outer wall 40, and the storage space 52 is partitioned and formed between both walls 30 and 40, and when the single sliding door 20 is opened, the door is pulled in the storage space 52. The outer wall 40 is freely fittably and extractably formed to the fixed inner wall 30, and guide rails 17 and 18 are arranged as a wall guide member in an upper frame 11 and a lower frame 12 fitted with the outer wall 40. The outer wall 40 forms grooves 41 and 42 fitted with the guide rails on an upper end surface and a lower end surface, and the outer wall 40 fitted in from the opening part 50 of the single sliding door 20 can slide in the opening-closing direction of the single sliding door 20, and an intermediate frame 43 is releasably fixed as a stopper on an opening part side end surface of the outer wall 40.