Abstract: A cutter knife with a blade extendable from and retractable into a cutter knife body by operating a slider. The cutter knife body has a plurality of engagement indentations formed alongside a slit. The slider has a spring member with an engagement head which is engageable with one of the engagement indentations. When an external force is directly applied to the blade in an advancing direction, the blade is inhibited from sliding out, because the distal side of the engagement head comes into contact with the distal end wall of the engagement indentation. When a predetermined external force is directly applied to the blade in a retracting direction, the blade can be withdrawn and held inside the cutter knife body by this external force, because an escapee is provided to the proximal end wall of each engagement indentation, or to the proximal side of the engagement head.