It is well discussed about biological effect to high-level radioactive waste (HLW) disposal and known that the biofilm is considered to be the uncertain factor to estimate for migration of radioactive elements. The objective of this research is to estimate the microbial effect of Cs migration in groundwater interacted with rock surface. Specially, we focus on Cs behavior at the rock surface surrounded by biofilm. The most important factor is the Cs sorption and diffusion to the microbe and/or their biofilm. Generation of bio-colloid absorbed with Cs and retardation of Cs by their matrix diffusion in rock will be influenced by these phenomena. We introduce about scenario analysis for biofilm and a simple Cs diffusion test with and without sulfur reducing bacteria (SRB) which is well known as easy to produce biofilm on the rock surface in order to clarify the existence effect of the bacteria at the rock surface. The Cs diffusion experiment, using Desulfovivrio desullfuricans as SRB, indicated that microbial effect was less to through their biofilm in the experimental condition. We consider that Cs is easy to contact the rock surface even if surrounded biofilm and not effect to retardation by matrix diffusion scenario.