To provide a method for preventing any uneven thickness of a seamless steel tube.

In the uneven thickness preventive method of a seamless steel tube, the amplitude of a mandrel during the drilling is measured in a seamless steel tube manufacturing method, and if the measured value is deviated from a predetermined control value, the deviation is searched by alarm, and the mandrel is replaced in a seamless steel tube manufacturing method. In the uneven thickness preventive method of the seamless steel tube, the amplitude of the mandrel during the drilling is measured by using a laser displacement gage. The number of times of service is increased by 1.3 times, and the fraction defective of uneven thickness (the fraction defective of weight) is reduced from the conventional value of 0.12% to 0.07%, and the level of uneven thickness is improved.