PURPOSE: An inside diameter adjustable ring is provided to enable a user to easily expand and reduce the inside diameter of the ring by inserting an insertion unit into a fixed unit and moving the insertion unit outside. CONSTITUTION: An inside diameter adjustable ring comprises insert units(2,3), a fixed unit(1), and hanging protrusions(4,5). The insertion unit is formed in both ends of the body. The fixed unit is formed in the center of both ends of the body. In the insertion unit of both ends, the hanging protrusions are projected to the right and left outside. In the inner sidewall of the fixed unit, a plurality of hanging protrusions(6) are installed to face each other. The insertion unit is inserted into the fixed unit and is moved outside in order to adjust the inside diameter of the ring.