To provide a shutter having a structure capable of preventing opening of the shutter from the outside irrespective of a brake condition.

An end slat 56 and a next step slat 57 on the end slat 56 are relatively connected so as to move vertically and freely. When a shutter is closed fully, a clearance G between both slats 56 and 57 is reduced more than in a hoisting down condition. A locking device is provided in at least either of the end slat 56 and the next step slat 57. The locking device protrudes on a guide rail side when the shutter is fully closed and is locked in a locking part formed on the guide rail. A lock device 90 which has an operation means on indoor side and locks while the clearance G between the end slat 56 and the next step slat 57 is kept small is provided.