A traction drive tool adapter is adapted to detect an overload on a planetary roller speed increasing mechanism which occurs due to an excessive depth of out, and prevent the breakage of the planetary roller speed increasing mechanism and the imperfect processing of a workpiece. It consists of a rotary portion, and a tool mount portion rotatable with respect to the fixed portion and rotary portion and adapted to attach a tool to a front end section thereof, all of these three portions are put together via a planetary roller speed increasing mechanism. The traction drive tool adapter is further provided with a first revolution speed detecting means provided on the fixed portion and adapted to detect the number of revolutions per unit time of the rotary portion, a second revolution speed detecting means provided on the fixed portion and adapted to detect the number of revolutions per unit time of the tool mount portion, and an abnormal rotation judgement means which is connected to the first and second detecting means so that output signals from these detecting means are input thereinto, and which is adapted to generate an abnormality signal when a speed increasing ratio, is lower than a predetermined level.