PURPOSE: To adjust backset of a deadbolt by installing a bolt train for adjusting the length of a movable housing by the use of a pin and a locating groove and a shuttle to hold in engagement the bolt train during adjustment.

CONSTITUTION: An adjusting plug 22 provided with grooves 27a and 27b and a notch 27c is installed slidably in a deadbolt 21, and a plug pin 24 to be engaged with the grooves is installed to make it possible to adjust the length. A bolt train driver 23 is engaged with the plug 22 and make it possible to rotate in a single piece. A handle H is installed, and it is made practicable that the deadbolt 21 is set in the extension position when the handle H is rotated. A housing 11 incorporating them is installed inside another housing 16 slidably. A shuttle 32 holds the deadbolt 21 during the adjusting operation. The housigs are rotated and the plug 22 is moved, and the members are fixed in cooperation. Thus a deadbolt backset adjusting mechanism is accomplished.