Auto-locking and step-sliding type cutter knife

A cutter knife comprising:

• an elongated sleeve member having a slit extending along a longitudinal direction, and a plurality of engagement indentations formed at predetermined interval alongside the slit;
• a blade slidably held inside the sleeve member;
• and
• a slider composed of a slider body which is connected to the blade and is slidable inside the sleeve member, an operating member which is so coupled with the slider body as to shift relative to the slider body and which is exposed to an external through the slit, and a spring member having a convex engagement head,
• characterized in that
• the spring member is held between the slider body and the operating member with its engagement head being urged into one of the engagement indentations of the sleeve member,
• when the operating member of the slider is manipulated to slide the blade, the operating member is shifted relative to the slider body to thereby put the engagement head of the spring member outside the engagement indentation;
• while when the engagement head is moved to another adjacent engagement indentation, the engagement head is urged into the another adjacent engagement indentation by an urging force of the spring member,
• when an external force is directly applied to the blade in the advancing direction of the blade, a distal end wall of the engagement indentation contacts with a distal side of the engagement head to inhibit the blade from sliding out, and
• when a predetermined external force is directly applied to the blade in the retracting direction of the blade, an escape provided to a proximal end wall of the engagement indentation allows that the blade is retracted into the cutter knife body.