To solve the problems with chuck mounting jigs of the prior art that they are cumbersome to install leading to poor productivity and that accuracy is bad since long T-groove bolts need be used to fix them.

A chuck mounting jig 4 for mounting a chuck 3 for retaining a workpiece on a table 1 having T-grooves 2 therein comprises a base plate 5 interposed between the table 1 and the chuck 3, chuck connections 6 provided on the base plate 5 in such a way as to correspond to clamping holes 3a provided in the chuck 3, and table connections 7 provided in the base plate 5 in order for T-groove bolts 8 mounted in the T-grooves 2 in the table 1 to be fitted therein. The mounting jig 4 can secure the chuck 3 to the table 1 by bolting together the chuck connections 6 and the clamping holes 3a in the chuck 3, and fitting the T-groove bolts 8 of the table 1 into the table connections 7 and fastening them together.