[en] This paper deals with a method of implementation of strain-rate dependence effect of elastic-plastic materials into a finite element code. The strain-rate dependence of yield surface is analyzed based on the viscoplasticity theory, in which two kinds of yield surfaces are used. Among the two surfaces, one is quasi-static yield surface, which expands its radius due to classical plasticity theory, and the other one is dynamic yield surface, which is the expansion or shrinkage of the corresponding quasi-static yield surface. The amount of the change from the quasi-static yield surface depends on the magnitude of the strain-rate at that current time. Strain-rate is calculated from the velocity at the node. This method enables us to deal with the phenomenon of the shrinkage of yield surface due to the sudden drop of strain-rate within the framework of plasticity theory. In this analysis, the strain-rate history effect is neglected. The analysis is also confined currently within the range of small strain. This method is implemented into the implicit and multipurpose finite element code ADINA. Two examples of calculated results are given, one is an impact of a bar against the rigid wall, and the other is an expansion of a pipe due to the explosive pressure. (orig.)