To automatically exchange a large-sized and heavy cutter.

In a machine tool 1 including a pair of work heads 2 for clamping both ends of a shaft-like work 3 to be machined with a chuck and rotating the same, and a cutter head 5 provided in such a manner as to freely moved in the longitudinal direction (direction of X-axis) of the work 3 and in the approaching and separating direction (direction of Y-axis) and adapted to milling the work 3 with an external blade type cutter 7 rotated by a spindle motor 6, the automatic cutter exchanger is formed by a loader 8 having a work holding means 17 for holding the work 3 which is carried in and out of the machine tool 1, a cutter holding means 18, which is provided on the loader 8, for holding the cutter 7 removably provided on the spindle 5a of the cutter head 5, and a control means for controlling the carry in-and-out operation for the work 3 and the removing operation for the cutter 7. By this device, a large-sized and heavy external blade type cutter 7 can be automatically exchanged safely in a short time so as to improve productivity.