A novel guide or jig for safe and accurate placement of fixation screws across a sacroiliac joint uses the tip of a sacro-pedicle targeting screw inserted from the rear into a human sacrum as a target for guiding fixation screws. The head of the targeting screw attaches to one end of a straight first arm of a jig member. The other end of the straight arm of the jig member is attached at an angle to a semicircular second arm of the jig member. The semicircular second arm has an open slot to which is attached a bushing assembly which can be moved along and fixed in place anywhere along the slot. A cannulated pin set fits through the bushing assembly. The dimensions of the targeting screw and the jig member, including the angle between the two arms of the jig member and the radius of curvature of the semicircular second arm, are such that the cannulated pin set, when inserted through the bushing assembly, lies along a line drawn as a radius from the tip of the targeting screw. The cannulated pin set is used to guide and insert fixation screws across the sacroiliac joint. The jig insures that at any position of the bushing assembly along the semicircular second arm a fixation screw will be properly guided into position across the joint.