Blow molding mechanism and mold unit

A mold unit comprising:

• a blow molding mechanism,
• wherein the blow molding mechanism includes
• an extension rod that is inserted into an intermediate molded product, which is molded into a bottomed cylindrical shape and includes an opening portion, via the opening portion and extends the intermediate molded product when blow molding is performed on the intermediate molded product,
• the extension rod includes a main body that has a columnar shape and extends inside the intermediate molded product and a tip part that is formed at a tip portion of the main body and includes a contact portion which is brought into contact with an inner wall of the intermediate molded product, and
• a diameter of the contact portion is larger than a diameter of the main body.