Abstract: An antitheft device comprises a tag 10 and a member 2 for attaching the tag to a commodity. The attaching member 2 comprises a button 22 and a pin 21 projecting therefrom. The tag 10 includes a housing 1 having arranged therein a clamp member 3 for clamping the pin 21 of the attaching member 2, an on-off switch 8 to be depressed by the button 22 of the attaching member 2, and a buzzer 6 operable under the control of on-off signals from the switch 8. The clamp member 3 is rotatably supported and disengageable from pin 21 when the clamp member 3 is rotated in one direction. The on-off switch 8 comprises an actuator 82 projecting outward from the housing 1, and a cylindrical guide portion 83 provided around the base end of the actuator 82 and serving as a pivot for supporting the clamp member 3 thereon. The device is adapted to reliably prevent unlawful acts without becoming large-sized.