[en] Instability phenomena of circular cylindrical shells due to annular leakage flow was studied both experimentally and analytically. In the analysis, an analytical evaluation method was developed using FEM which allows to treat the non-uniform longitudinal distribution of the shell stiffness of two elastic coaxial circular cylindrical shells. A wave equation was derived based on the velocity potential theory for both the annular flow and the inner flow inside the inner shell assuming that the fluid is inviscid, irrotational and compressible. Applying the coupling conditions of the shell and fluid, a coupled equation of motion was obtained. Instability was identified by complex eigen value analysis. Divergence, one of the instability phenomena, was predominantly found in both the experiment and analysis, and the effects of shell rigidity, length and annulus gap were systematically investigated and discussed. (author)