PURPOSE: To equalize gaps between respective members in a welded and assembled car body by determining any one of respective reference holes for mounting doors, front fender and bonnet as an original reference one to subsequently work to two remaining reference holes.

CONSTITUTION: Common reference holes 6A, 6B for mounting a front fender 5 and bonnet in the front section of a car body are worked subsequently in the car body 1 with reference to a reference plane having a door mounting reference hole 3 for mounting a door 2 to the car body 1. To work the reference holes 3, 6A, 6B, abutting pins 18, 19 of a base frame 11 are first adapted to abut against the upper surface of a welded and assembled car body 1 and locate supportingly a base frame 11 to the car body 1. Then, a movable frame 14 is translated in the directions X, Y relative to the base frame 11 so that an abutting part 21a of an abutting member 21 abuts against and engages the reference plane 4. The reference holes 6A, 6B are worked by drilling the welded and assembled car body 1 through guide parts 22a, 23a of guide members 22, 23 under such condition.