An object is mounted and rotated on a rotary table. An encoder outputs a rotary angle signal corresponding to the rotary angle of the rotary table. A detection head outputs a displacement detection signal while tracing the surface of the object. The detection head moves within a plane including the axis of rotation, so as to oppose the outer or inner surface of the object. When the detection head is opposed to the inner surface of the object, the positive and negative signs of the displacement detection signal is reversed, while at the same time a 180° angle is added to the rotary angle signal. By utilizing the reversed displacement detection signal and the corrected angle correction signal, the obtained result can be displayed through a displaying processing common to the processing for the outer surface.