a separable rotary tool adapter consists of a low - speed rotary unit having casings ; a first rotary shaft in the casing connectable to a driving shaft of the machine tool and a second rotary shaft orthogonally drivingly connected to the first shaft ; a high - speed rotary unit having a casing detachable to the casing of the low - speed rotary unit ; a rotary shaft in the casing of the high - speed unit having a free end portion connectable with a rotary tool ; and a planetary roller mechanism having two parts engageable with and disengageable from each other in the axial direction , each part being cooperatively associated with one of the high - speed rotary shaft and the second rotary shaft . in the low - speed rotary unit , a first part of the planetary roller mechanism is attached to the second rotary shaft engaged with the first rotary shaft at right angles via a transmission mechanism and includes planetary roller driving retainers or shaft pins . in the high - speed rotary unit , a second part of the planetary roller mechanism is engageable with and disengageable from the first part . the separable rotary tool adapter in which the low - speed rotary unit and high - speed rotary unit are unitarily combined is mounted on a rotary tool mounting portion of a turret in a machine tool , e . g ., a turret lathe .