Casing treatment is known to improve the stall margin of axial compressors. However, it is known as well that casing treatment lowers the efficiency of compressors. An experiment was planned in order to test a series of configurations of casing treatment which are supposed to have favourable effects and to obtain the optimum ones, if any, judged from above-mentioned two standpoints: larger stall margin improvement and smaller lowering of efficiency. The result shows that the relation between the stall margin imp improvement and the compressor efficiency for all of the tested configurations falls on some smooth curved-line, and thus, there exists no particularly superior treatment configuration to others. It rather shows that a certain amount of loss in efficiency is inevitable in order to obtain the required amount of stall margin improvement. Influence of rotor tip clearance on the effect of casing treatment was also examined. The result can be used as a guide for the selection of configurations in designing the casing treatment.