PURPOSE: To make improvements in the operation efficiency of a self-propelled saw, by making up each position of a plate material and a rail so as to simply and accurately be carried out when these plate material and rail are slung down by a crane, in time of cutting by means of the self-propelled saw.

CONSTITUTION: After an aluminum plate material 13 subject to cutting is mounted on a frame 11, positioning for a cutting jig 15 provided with a rail 14 for a self-propelled saw 14 is carried out, then the saw 14 is made to travel along the rail 14, and this plate material 13 is cut off by a saw blade 4A. Upon this, when the plate material 13 of, for example, a cryostat zone plate or the like of a spherical tank for a liquefied natural gas (LNG) tanker and the jig 15 are slung down with a crane or the like, three positoning jigs 16 for their location are installed on the frame 11. And, the rail 14 for the jig 15 slung by the crane are downed as contacting a guide 3, and positioning for the rail 14 takes place. Thus its operation efficiency can be improved.