To cut the lower edge of a cloth on the wall at a prescribed position more precisely, holding a retractable knife one-handedly.

This retractable knife 10 is suitable for cutting a cloth sheet adhered to one of the planes orthogonally crossing with each other along the crossing line of the two planes. A first guide part 28, whose section is shaped so that it can be in contact at points with both planes 50 and 51 orthogonally crossing with each other, is attached on the bottom of the base part of the retractable knife 10 which a user can hold by one hand. The top of the cutting edge 25 of the knife is made to protrude downward slantingly from the first guide part 28 at the center of the first guide part 28 in the direction of thickness, keeping the posture of the cutting edge against the cloth sheet to be cut.