To reduce burrs formed on both cut surfaces after cutting a lightweight shape steel.

A cutting tool 1 is provided with a knife blade 3, a blade receiver 2 which is placed at the forward side of the knife blade 3 to receives the knife blade 3 and hold a lightweight shape steel, and handle parts 4a and 4b for driving the knife blade 3 toward the blade receiver 2. The knife blade 3 has forwardly protruding blades 20, 21, 22, and 23, first blade tips which are formed at tips of the protruding blades 20, 21, 22, 23 so as to extend in a direction of the knife blade thickness, and is arranged correspondingly to a curved portion of the lightweight shape steel so as to be brought into close contact with the curved portion and start cutting, and a second and a third blade tips which extend from both ends of the first blade tip to a width direction of the knife blade 3 and are each formed separately in a direction of the knife blade thickness.