A combustor-transition-piece guide jig is constituted by two rails, a fixing member, and a holding member. The fixing member is provided at one ends of the rails and attached to a combustor attachment port. The rails are inserted from the combustor attachment port toward inside of the combustor casing and fixed to the combustor attachment port by the fixing member. The rails come into contact with a combustor-transition-piece guiding part provided on a combustor transition piece to guide a movement of the combustor transition piece in one direction.