To correctly estimate and identify the causes of thickness non-uniformity, and to take an appropriate thickness non-uniformity preventive action in an early stage in manufacturing a seamless pipe.

In a thickness non-uniformity monitoring method of the seamless pipe, the condition of generating the thickness non-uniformity is monitored by continuously measuring the vibrating condition of a bored or rolled stock pipe by using a two-axis CCD camera arranged orthogonal to each other on the outlet side of inclined rolls in the case of manufacturing the seamless pipe obtained by boring or rolling the stock pipe by a pair of the inclined rolls and plugs located between the inclined rolls. Further, in the thickness non-uniformity monitoring method of the seamless pipe, the measurement value is Fourier-transformed to estimate the causes of generating the thickness non-uniformity from he periodicity of the vibration. Still further, in the thickness non-uniformity monitoring method of the seamless pipe, the causes of generating the thickness non-uniformity is identified based on the measurement value obtained by continuously measuring temperature distribution in the longitudinal direction of the stock material or the stock pipe and/or the load distribution of the inclined rolls.