PURPOSE: To provide a locking structure for an automatic tool. exchange device, which enables locking to be completed at a time when a tool exchange arm is rotated to a hold position, prevents a tool from being inclined even if it is heavy, and also enables locking actions to be surely carried out.

CONSTITUTION: The device is provided with an energizing piston 15 enegizing a movable grip 13b to a hold position, and with a lock bar 16 energized to a lock position which prevents the energizing piston 15 from retreating back, so as to be mounted on an exchange arm 12. The device is also provided with an unlock surface 17a which is mounted on a base member in such a way as to be rotatable around an axis parallel with an exchange shaft 11, and moves the lock bar 16 to an unlock position, and with a switching block 17 including a lock cut-out section allowing the lock bar to be moved to the lock position. A drive pin 20 is provided, which allows the switching block 17 to be rotated to the lock position with the exchange arm 12 rotated to a tool hold position.