To provide a mounting jig to allow a member to be mounted, such as a seal ring to be fitted correctly and certainly in a, groove furnished at the peripheral surface of the member on which mounting is to be made such as a piston.

The mounting jig 1 for a member to be mounted 23 such as a seal ring to be fitted in a groove 22 furnished at the peripheral surface of a member 21 on which mounting is to be made such as a piston is equipped with a guide jig 2 provided with a tapered guide surface 4 to enlarge the member 23 gradually in association with its sliding and an intruding jig 11 provided with an intruding piece 13 for sliding the member 23 along the guide surface 4, wherein the length L from the tip 2a of the guide jig 2 to the groove 22 in the condition that the guide jig 2 is mounted on the mounting member 21 is arranged equal to the depth H1 of the intruding jig 11.