Abstract: A backset adjusting device of a cylindrical door lock. This device free adjusts the backset of the door lock between the 60 mm backset and the 70 mm backset and lets the door lock be commonly used in the conventional doors regardless of the door types. A latch bolt extension slide coupled to a latch bolt has an axial guide slot. A retractor slide is coupled to the latch bolt extension slide so as to linearly reciprocate with respect to the extension slide by 10 mm. This retractor slide has an opened axial guide slit at its front section and a side guide projection movably received in the guide slot of the extension slide for guiding the axial reciprocation of the retractor slide with respect to the extension slide. A pair of hooking notches are formed on the rear section of the retractor slide. An outer cylinder is movably fitted over a rear section of an inner cylinder and coupled to the hooking notches of the retractor slide.