PURPOSE: To prevent deviated load on a truck for a launching work, by providing a partial launching line rotatably at a horizontal end portion of a launching line, mounting a caisson on the truck, firstly moving the caisson and the truck with the partial launching line held horizontal, then rotating the same.

CONSTITUTION: A truck 13 for mounting a caisson A thereon is disposed movably on an oblique launching line 12 extending from a caisson manufacturing yard on a land into water, so as to carry out a caisson launching work. A partial launching line 10c is provided at a horizontal end portion 10a of the launching line 12 so as to be rotated by a hydraulic cylinder 17 and the like. The truck 13 mounting the caisson A thereon is moved to the partial launching line 10c rotated to a horizontal position, and then the partial launching line 10c is rotated to coincide with an oblique line 10b and launch the caisson. With this constitution, since a concentrated load of the caisson A is not partially applied on the truck 13, it is possible to prevent the caisson A from being damaged.