To provide a crime prevention unit for a building, enabling precaution against illegal opening of an opening portion even when the opening portion is opened for ventilation.

An openable window unit 13 is provided at an opening portion 12 formed on a partition wall 11 of the building. The opening portion 12 is opened or closed by paned windows 15, 16 of the above openable window unit 13. Also, a crescent key 17 is mounted on the paned windows 15, 16, and by means of the above crescent key 17, the paned windows 15, 16 are locked. In the above configuration, a lock/unlock detection sensor 21, a microcomputer 23, etc. are incorporated in a crescent handle 17b of the crescent key 17. Further, a rotation number detection unit 31 is incorporated in the window sash 16a of the paned window 16. Based on the detection result by the lock/unlock detection sensor 21 and the rotation number detection unit 31, the microcomputer 23 takes precautions on the state of the opening portion 12.