To provide a collapsible utility knife capable of easily folding up a blade in a handle, capable of firmly holding the blade, easy in its replacement, easy in use, and requiring no high cost for manufacture and maintenance.

This utility knife 1 has the handle 2, and a blade holder 3 revolvably installed on the handle and movable to a folding-up state from a developing state. The handle has a space part capable of storing the blade holder when the blade holder exists in its folding-up state. The blade holder includes a main wall 34 and a guard wall 35 installed together and capable of storing the blade B between its walls, and a lock means revolvably installed in the blade holder and locking the blade in a fixed position.