To prevent the blade of a cutter from being unpreparedly broken, make easy to add a force, and improve the workability by fitting a cover guide to the part of the exposed blade of a cutter knife so as to cover the back side of the blade of the cutter knife.

This replaceable blade type cutter knife 1 has a cover guide 10 about on the straight line passing the working point of the cutter. In use, a cutter blade 2 is set so as not to protrude from the cover guide 10. By forming the cover guide 10 in this way, the cutter blade 2 can be held and stabilized. Accordingly, the cutter blade 2 is strong to a horizontal force and hardly broken. Even if a force is added from above the straight line passing the working point of the cutter, the back part of the exposed replaceable blade is never directly pressed, and the damage of a finger can be thus prevented. The material of the cover guide 10 is not particularly limited, and the form or size thereof can be also optionally selected. The fixing method is not particularly limited except its fixing to the horizontal direction of the cutter blade 2.