The present invention relates to a cap loosening prevention device for preventing a cap from loosening when the cap is fitted to a body, the device including a body ring, which is formed in a circular shape along the circumference of the body and has a cut portion, and having a protruding part formed on the outward surface of the body ring, wherein the protruding part has an incline with respect to the outward direction of the body ring, and the uppermost end of the protruding part is cut in a perpendicular direction with respect to the outward direction of the body ring. According to the present invention, the cap is prevented from completely loosening in the direction opposite to the rotational direction of the cap in a state in which the cap is fitted to the body, such that the cap can be firmly coupled to the body so as not to be separated from the body, and the rotation of the cap is minimized so as to enable a rapid operation when a pipe is replaced.
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