To make a cutter knife usable with a lot of practicality and safety by providing a means for sliding a blade to a projecting position, a means for biasing it in a backward direction and a means for locking the sliding of the blade at the projecting position in a cutter knife having a holder main body for holding the blade freely slidably.

When the cutter knife is used, the finger is put on a projection 32 which is projected from the sliding groove 11 of the holder main body so as to push it forward against the spring force (bias force) of a coil spring 5. Then, a slider 3 is moved forward along the sliding grooves 10A and 10B and the blade 2 which is fixed to the slider 3 by a temporary fixing part 22. Besides, the slider 3 is slid and the tip (lock part) 34 of a leaf spring-shaped stopper 33 which is fixed to the slider 3 comes into slidable contact with the inner wall surface of a right side member 1B in a bending state. When the slider 3 reaches a position corresponding to the projecting end of the blade 2, the lock part 34 is fitted into a hole 12 and locked with an impact on the head part rear surface of a lock release button 4 and the blade 2 is locked at the projecting position.