To provide a method for measuring the outside diameter of a seamless steel tube capable of obtaining the highly reliable measurement value even when the temperature at the measurement is changed.

In the method for measuring the outside diameter of the seamless steel tube, a convex mirror and a CCD camera face the steel tube with the predetermined distance separated from the seamless steel tube traveling on the manufacture line in the axial direction, the image of the steel tube is picked up by the CCD camera, then the threshold is set to the signal level of the light received by light-receiving elements arrayed in one row in the camera, and the position of the light-receiving element corresponding to the threshold is measured as the outside diameter of the steel tube. More in detail, the threshold is changed according to the temperature at the measurement of the seamless steel tube.