A machine tool includes a machine tool main body, a ram which is supported with respect to the machine tool main body in a movable manner, a main shaft which is supported by the ram in a drivable and rotatable manner, an attachment which can be attached to and detached from a tip end portion of the ram and includes a driving shaft rotated according to rotation of the main shaft and a tool provided on the driving shaft, and a NC device which performs a numerical control based on machining data, and performs machining of a processing target. The control method of a machine tool includes a step of decreasing at least one of a ram overhang amount or a feeding amount when stress applied to the attachment is larger than allowable stress of the attachment based on a machining condition including a diameter, a depth of cut, and a feeding amount of the tool, and information including the ram overhang amount, a shape of the attachment, and a material of the processing target.