the holding cylinders 88 and 89 respectively the urging pins 92 and 93 are biased by springs 94 and 95 contained within the corresponding holding cylinders 88 and 89 respectively with such arrangement the support jig 8 is urged by the urging pins 92 and 93 with the upper and lower support projections 84 and 85 placed on the protrusions 90 and 91 on the side of the jig carrier 64 and is supported on the jig carrier 64 with the stoppers 86 and 87 abuting against the corresponding protrusions 90 and 91 if the jig carrier 8 is brought into the working station s in such state then the axes of the support holes 78 83 and 79 in the support jig 8 are off of the axis of the boring bar 7 located above them and for the boring it is necessary to move and fix the support jig 8 closer to the jig carrier 64 when the support jig 8 is positioned at the working station s a jig clamp cylinder 96 and a jig clamp cylinder 97 are arranged on the side portion of the post 3 at the upper and lower portions of the jig body 75 respectively the jig clamp cylinders 96 and 97 each have a.