PURPOSE: To enhance performance efficiency and its outside view by installing a pair of bars fixed with the roof of an automobile with a stay and a ladder having a means designed to make a parallel movement in the direction of both ends of the bars supported with the bars and a means which temporarily supports the ladder and the bars. CONSTITUTION: A pair of bars 2a and 2b are mounted to stays 1a to 1d installed on the roof of an automobile. Then, projected parts 4a to 4d installed outside posts 5a and 5d of a ladder are fitted into a groove 3b installed inside the bars 2a and 2b. Projected parts 9a and 9b of posts 10a and 10 are fitted into recessed parts 6a and 6b installed to the posts 5a and 5b. When it is necessary to use the ladder, a handle 7a or 7b mounted to a step 8a or 8b is rotated outside and upward so that two projected parts 4a and 4b or 4c and 4d on one side of the ladder may be pulled into the post 5a or 5b and forced to slide through their rotary motion and interlocking motion. This construction makes it possible to mount a ladder to be exclusively used for roofs the roof of an automobile whose height is high with high efficiency.