To make it extremely easy to construct a large and heavy door (a) by enabling easy engagement of a lower runner 6, as it is, with a traveling channel 4a from an opening part 4b of a lower rail 4, by forming an equal clearance X between each of outer end edges of both right and left engaging parts 6a, 6a of the lower runner 6 and each of inner end edges of the opening part 4b of the lower rail 4 in the state that the door (a) is arranged upright.

Engaging grooves 4c, 4c are formed in the inner depths on right and left sides of the opening part 4b of the lower rail 4 in a communicating manner. The right and left engaging grooves 4c, 4c are formed so that the engaging parts 6a, 6a projecting from both of right and left sides of the lower runner 6 fitted to the door (a) can be engaged with them. The width of the lower runner 6 at the two right and left engaging parts and the width of the opening part 4b of the lower rail 4 are set so that the equal clearance X may be formed between each of the outer end edges of the two right and left engaging parts 6a, 6a and each of the inner end edges of the opening part 4b of the lower rail 4, in the state that the door (a) is arranged upright.