Extension-retraction device

An extension-retraction device comprising:

• an extendable and retractable cylindrical structure;
• and
• an inner guide member disposed inside the cylindrical structure, wherein:
• the cylindrical structure includes an outer belt and an inner belt disposed on an inner side of the outer belt;
• the outer belt and the inner belt are each provided with a plurality of protrusions and a plurality of holes provided in two rows in a longitudinal direction;
• the inner guide member includes a first spiral groove extending along protrusions provided in one of the two rows on a distal end side of the cylindrical structure, and a second spiral groove extending along protrusions provided on the other of the two rows on a base side of the cylindrical structure;
• when the inner guide member rotates in one direction, the outer belt and the inner belt are sent out to be spirally wound while engaging with each other, and the cylindrical structure extends;
• when the inner guide member rotates in another direction, the outer belt and the inner belt that have been engaged and wound are drawn in to be unwound, and the cylindrical structure retracts;
• the first spiral groove includes a fitting portion in which one or more of the protrusions of the inner belt are fitted in the first spiral groove;
• a first straight line connects an end portion of the fitting portion and a central axis of the cylindrical structure;
• a second straight line connects a terminal end of the second spiral groove and the central axis of the cylindrical structure;
• and
• on a plane orthogonal to the central axis of the cylindrical structure, an angle made by intersecting the second straight line and the first straight line is less than 180 degrees.