A machine tool improving measurement accuracy of a machined workpiece diameter. A machine tool comprises a displacement sensor mounted on at least one of a guide bush and a spindle supporting unit. The machine tool further comprises a calculating unit which calculates a diameter of the machined workpiece based on a measurement value of the machined workpiece by the displacement sensor. The spindle is movable back and forth in an axial direction. The spindle is retracted after machining to bring a machined portion of the workpiece to a predetermined measurement position in the axial direction to allow the displacement sensor to measure the machined portion.