PURPOSE: To reduce time for the exchange of cutters, improve working efficiency and ensure safety in work by exchanging automatically the cutters for processing the side edge of a steel strip on a rolling line using a retaining claw fitted to a bogie travelling over a cutting head.

CONSTITUTION: In the exchange of a milling cutter 16, a working platform 30 is used and an overhead crane is operated for carrying a new cutter to the preparation part 23b of a cutter bed 23 for the exchange of an old cutter, with a plate edge cutting device under operation. And a travelling bogie 25 is moved to a position over a cutting head 14, the old cutter is dismantled from a spindle 17 fixed with a retaining claw and then the bogie 25 is moved to a position above the temporary stowage part 23a of the cutter bed 23 for placing the new cutter thereon. Then, the bogie 25 is moved to a position above the preparation part 23b, the new cutter is held with the retaining claw and the bogie 25 is moved to the cutting head 14 for mounting the new cutter to the spindle 17. And the old cutter is shifted from the temporary stowage part 23a and placed on the preparation part 23b.