PURPOSE: To enable to cut a V-groove with high accuracy, by a method wherein a coolant is supplied on the forward side of the moving direction of a cutter at the time of downward cutting and on the backward side of the cutter at the time of upward cutting, in cutting a V-groove by a milling machine.

CONSTITUTION: A coolant nozzle 11 having a tip part bifurcated is provided for a double angular cutter 3 fitted to a main spindle 10 of a vertical-type milling machine. For downward cutting, the nozzle 11 is placed on the forward side of the cutter 3, and a coolant is supplied to a part in proximity to a hatching part. On the other hand, for upward cutting, the nozzle 11 is placed on the backward side of the cutter 3, and the coolant is supplied to a part in proximity to the hatching part. Accordingly, by adjusting the coolant nozzle 11 to an appropriate height, the coolant can be evenly supplied to upper and lower cutting faces of the cutter 3, and can be made to act effectively, so that a uniform V-groove can be cut.