[en] The leaching behavior of ¹³⁷Cs has been studied to evaluate safety of sea and ground disposal of the cement composites. The rate depends on flow rate of the external solution, particle radius and composition of the cement composite. The rate-determining step of the leaching in the dynamic condition is the internal diffusion through the matrix cement composite. The rate in the static condition, on the other hand is controlled by external diffusion through the interface layer between solid and liquid. The cement composites containing mineral zeolite(25%) give very low leachability; the leaching fraction is 0.001 - 0.02 for the portland cement and 0.001 - 0.002 for the slag cement. (auth.)