To prevent a back door from vibrating to generate a large release sound when engagement is released between a locking member and a striker, in an automobile provided with the back door supported in a turnable and openable/closable manner to an automobile body, a striker fixed to the back door, and a locking device provided at the automobile body, wherein the locking member of the locking device is engaged with the striker to lock the back door in the totally closed position when the back door is in the totally closed position.

Reinforcing members 21, 22 for suppressing vibration generated at the back door 2 by an impact received by the back door 2 when engagement between the striker 13 and the locking member 18 is released is fixed to the back door 2.