In the paper, a theoretical and experimental investigation of a new type of the in-line optical fiber ellipsometer is described. The discussed device, based on the Sagnac interferometer, has the possibility to detect the changes of full polarisation state. The detection of the polarisation state in real time by a system containing standard single-mode fiber and an appropriate applied modulation technique is a new system property. The device uses interferometric measurement technique based on the fourth Fresnel-Arago's condition, which secures very good system accuracy and stability, also presented in the paper.