A balloon attaching jig is constituted by a ring member and a pair of nail members. After the insertion of the pair of nail members into an attaching hole of a balloon, the leading end of an insertion portion of an endoscope is pushed into an opening of the ring member. The diameter of the pair of nail members is expanded by a pushing-in force of the leading end of the insertion portion generated during this pushing in. As a result of this, the leading end of the insertion portion is inserted into the attaching hole of the balloon and hence the balloon can be easily attached to the leading end of the insertion portion of the endoscope.