Abstract: In a method of and a system for unpacking bales each tied by at least one strap, a contact plate member provided with a strap sensor unit is mounted, through a biasing arrangement, on a body frame for movement relative thereto. The body frame is moved toward a confronting side face of the bale to bring the contact plate member into contact with the side face of the bale. The body frame is further moved toward the bale while increasing biasing force of the biasing arrangement, until contact pressure, with which the contact plate member is urged against the side face of the bale, reaches a predetermined value. When the contact plate member is urged by the bale in such a direction as to increase the biasing force during scanning of the strap sensor unit along the side face of the bale, the body frame is moved away from the bale to maintain the contact pressure at the predetermined value.