To provide a processing jig for oval holes to be engaged with a reference hole formed in a work to be processed, whereby an oval hole is formed by spreading the reference hole using a boring tool with the aid of the jig.

A projection 20 of this processing jig 10 for oval holes is inserted into the reference hole 26, whereby the center of the hole formed in the jig body makes displacement for a distance L corresponding to the separation between a circular arc part 22 inside the projection 20 and the central part of of a circular arc part 24 located outside. Thereby the unprocessed portion in a crescent shape positioned near the reference hole 26 appears in the hole. Then a machining blade such as a drill, hole saw, etc. is again inserted from the hole, and boring is made to the unprocessed portion.