Abstract: A thrusting force detecting device of a rotary machining tool includes a cylinder chamber formed within a ram for lowering and elevating a spindle of a cutting tool, a fluid chamber formed within the cylinder chamber and having a larger diameter than the cylinder chamber, a piston slidably and liquid-tightly fitted in the cylinder chamber and having a collar thereabout fitted in the fluid chamber, the collar dividing the fluid chamber into first and second chambers, the piston being rotatably mounted about the spindle in axially fixed relation thereto, means for supplying fluid pressure to the first and second chambers, and pressure sensitive means responsive to the pressure variation in the second chamber.