The invention provides a standardized machining jig to be used in a machining center for processing various works. A machining jig 1 mounted on an exchange table T.sub.1 of a machining center has a jig base 10 and a pallet 20, and the pallet 20 is equipped with a large number of even-pitched through holes. Standard bolts 30 are inserted to the through holes, which are prevented from falling to the floor by a stopper plate 80. Standardized supporting blocks 42, 43 and positioning blocks 50 are fixed to the pallet 20 via bolts 30. A work 100 is fixed to the supporting blocks and the positioning blocks. The bolts 30 are manipulated via a bolt runner 60 attached to a robot arm.