To prevent the natural drop of a door body when the tension is lost due to breakage of a hoisting body, and to close the door body in an overhead door to hoist the door body by the hoisting body.

A drop braking device 9 comprises a roller body 10 which is pressed and urged against a guide rail 5 in an attachable/detachable condition, a working arm 12 to keep the roller body in a separated posture from the guide rail 5 under the tension of a wire rope 7, and a viscous clutch 13 to give brake to the rotation of the roller body. A dropping door body is descended in a braked condition by rotating the roller body 10 pressed against the guide rail 5 in a braked condition.