To improve a crime preventive property facilitating the attachable/detachable operation of a prime preventive cover by a habitant and making it impossible to dishonestly unlock the thumbturn by an outsider.

The member has several protrusions 7 overhanging outside and a prime preventive cover 1 formed of a transparent dome-shaped part capable of covering the skirt part 5 in which a magnet 6 is embedded and the whole body of the thumbturn 2 are fixedly engaged to a circular prime preventive cover holder 3 formed of a notched part 9 corresponding to the protrusion 7, a space part 10 fixing the protrusion 7, and a doughnut-shaped metallic plate 8 part where the skirt part 5 of the prime preventive cover holder 1 in which the magnet 6 is embedded is grounded. In this way, dishonest unlocking such as the thumbturn rotation or the like can be prevented.