An automatic tool exchange system which allows a tool management mode to be arbitrarily selectable between a fixed address scheme and a memory random scheme in operation of the same machining center. The control unit for controlling the respective drive units of the tool exchanger and the magazine includes a storage unit for storing tool information containing a registered correspondence relationship between the tools and corresponding addresses of the stowage parts of the magazine, a modifying unit for modifying the tool information, and a tool exchange instruction unit for sequentially designating the tool to be used in each of the machining steps in accordance with the machining program, and driving the magazine to move the stowage part at the address corresponding to the designated tool to a predetermined position based on the tool information so that the tool is located at the exchange position at a predetermined timing. The control unit is configured to manage the tool in a mode selectively switchable between a first tool management mode in a fixed address scheme for returning the tool after use to a corresponding address, and a second tool management mode in a memory random scheme so as to stow the tool after use at an unoccupied address where the tool for next use has been previously stowed, and allow the modifying unit to modify the correspondence relationship between the tool after use and the address, which has been contained in the tool information.