To provide a cutter knife with a protrusible/retractable blade body.

In a slider 2, a locking spring 10 comprises a leaf spring 23 and a locking shaft 24 separably mounted on the leaf spring 23, and an operation body 9 is surrounded by a surrounding part 40. The leaf spring 23 is inserted into a lock chamber 13 from an operation hole 15, and the locking shaft 24 is inserted into the lock chamber 13 from the assembling port parts 14a of both upper and lower locking holes 14 and a part of the operation hole 15. The movement of the locking shaft 24 on the loaf spring 23 is regulated in the longitudinal direction A, the vertical direction, and the lateral direction C, respectively. When the moving force in the longitudinal direction A is applied to the slider 2 in a locked state PR, the component force of the force applied to a locking recess 7 from the locking shaft 24 becomes the exciting force of the leaf spring 23. The locking spring 10 can be easily manufactured. The locking spring 10 can easily assembled to a slider body 8. The locking shaft 24 can be stably supported on the leaf spring 23, and the locked state PR is hardly released inadvertently.