An object is to provide a balloon mounting jig capable of reducing the burden of an operation of mounting a balloon in an insertion part of an endoscope. A balloon mounting jig (100) for mounting a balloon (60) including a first sleeve part (60A), a second sleeve part (60B), and a balloon main body (60C), to an insertion part (12) of an endoscope (10) or an insertion assisting tool, the balloon mounting jig (100) including: a main body (102) formed in a hollow cylindrical shape to be folded flat and having a first opening part (104) and a second opening part (106); a pair of guide pieces (108a, 108b) that face each other and are provided in the second opening part (106). A sliding surface (120) on which an outer peripheral surface of the insertion part (12) or the insertion assisting tool is slidable is provided on inner surfaces of the main body (102) and the pair of guide pieces (108a, 108b).