ANTITHEFT DEVICE

An antitheft device, comprising a tag (10) and a product mounting member (2) , wherein the product mounting member (2) is formed by projectedly installing a pin (21) on a button (22), a clamp member (3) for clamping the pin (21) of t he product mounting member (2), an on/off switch (8) pushed down by the button (22) of the product mounting member (2), and a buzzer (6) operationally controlled based on the on/off signals from the on/off switch (8) are dispos ed in the housing (1) of the tag (10), the clamp member (3) is rotatably supported and releasable from the pin (21) according to the rotation thereof in one direction, the on/off switch (8) comprises a working piece (82) projected to the outside of the housing (1) and a cylindrical guide part (83 ) surrounding the base end part of the working piece (82), and the cylindrical guide part (83) forms the support shaft of a spring clamp member (3), whereb y an illegal practice can be surely prevented without increasing the size of t he device.