In the framework of activities dedicated to the GAIA mission, Alcatel Alénia Space has developed a High Stability Optical Bench (HSOB) made with CeSiC. A key criterion was the stability of its geometry, requiring it to be confirmed by tests [1]. The idea was to use a metrologic system able to work in a primary vacuum environment with sub-nanometric resolution and reproducibility compliant with the expected performances of the bench. In answer to these requirements, SAGEIS-CSO has proposed a system consisting of a frequency stabilised laser, an optical coupler, two optical sensor heads named MOUSE I and a detection unit. The delivered information is a longitudinal displacement measurement between the sensor and its joint retroreflector. Verification of the resolution by tests has given a value of about 15 pm in a 15 Hz pass band. Displacement repeatability versus temperature has been checked before tests campaign and in the final configuration, giving a value between ± 1.4 nm/K and ± 1 nm/K. These excellent performances allowed the sensor to qualify the stability of the HSOB.