Abstract: Herein disclosed is an exchange arm drive control system which is adopted as the automatic tool exchanger of a vertical quadruple spindle type machining center and enabled to contribute to sparing the space and improving the machinability. The drive control system is comprised mainly of means for clamping a tool and moving a tool holder in a Z-axis, means for rotating the tool holder by 180 degrees, means for turning exchange arms by 90 degrees and controlling the position of the tool holder; and means for moving the exchange arms on a Y-axis. Especially, in the 90-degree exchange arm turning and tool holder positioning means of those means, there is adopted a hollow shaft (16) for keeping a rotary shaft (2) away from associating with the 90-degree turn of the exchange arm (11) so that the tool can be conveyed with the tool holder being held in a constant position.