In a machine tool with an automatic tool change function, a rotatable tool spindle carries a key engageable with a key-way formed on a tool. A tool change arm, serving as a tool support device, rotatably supports at least one tool to selectively insert and remove the same into and from said tool spindle. When a tool change operation is to be performed, the rotation of the spindle is changed into a predetermined speed suitable for the key-engagement. An operating device is provided for causing relative movement between the tool change arm and the spindle to insert a tool into the spindle, however, the relative movement thereof is stopped by a stopping device just before the engagement of the key with the key-way. Subsequently, when a first detecting device detects that the key reaches to a predetermined angular position, the relative movement of the tool change arm is permitted to thereby make engagement of the key with the key-way. A second detecting device detects the key-engagement and then the tool is clamped in the spindle for a machining operation. After the machining operation, when a third detecting device detects that the key reaches to a predetermined angular position, the operating device is operated to move the tool change arm for removing the tool from the spindle.