A measurement system for measuring a small-sized hole with a 17 to 21 mm diameter and 1 000 mm length was constructed. The constructed system consists of a measurement probe, a laser interferometer for detecting the roundness of a hole, and an optical system for detecting the probe position and inclination. The probe consists of a rotatable measurement unit with a pentaprism and corner cube prism, a DC motor for rotating the measurement unit, and a laser diode to radiate a laser beam toward the optical system. In this study, the measurement accuracy of the system is examined through a comparison with other measurement instruments. Further, factors causing measurement errors are analyzed and discussed. As a result, it is clear that a deviation of the probe from the center of the hole and the fabrication error of the probe adversely affect measurement accuracy. Further, small deviations lead to a decrease in measurement errors and a simplification of the measurement system.