To provide a handheld instrument such as a medical knife, capable of simplifying the locking and unlocking operation and allowing the instrument to be easily retained in the locked state.

The handheld instrument comprises a holder 1, a movable head 21 to which a blade body 24 is attached, and an operating body 14 for supporting the movable head 21. The movable head 21 moves together with the operating body 14 relative to the holder 1, and takes the storage state or the projecting state separate from each other. In the holder 1, locking recesses 11 and 12 are juxtaposed in the direction X of movement of the operating body 14 on the front and rear sides of a guide aperture 10. The operating body 14 has a leaf spring part 17 for energizing an operating part 15 in the circumferential direction relative to an axis 16a so that locking projections 19 are locked with the locking recesses 11 and 12 of the holder 1. The lock of the locking projections 19 of the operating part 15 with the locking recesses 11 and 12 of the holder 1 can be released by operating the operating part 15 against the elasticity of the leaf spring part 17.