In this study, the role of fluids in a slow slip events (SSEs) regime was examined by investigating the rheological properties of the solid-liquid two-phase system on a laboratory scale. We specifically investigated how the rheological properties change with the shear rate when the granular layer is fluid-rich. We used a velocity-controlled rheometer to apply shear to a liquid-saturated granular layer. The results show that the liquid-saturated granular layer’s shear viscosity depended on the liquid viscosity and exhibited pseudoplastic behavior. Additionally, the saturated granular layered exhibited hysteresis, which increased as the viscosity of the liquid decreased. After a quantitative discussion through Bagnold (1954) framework, we propose a fault model that includes a fluid-rich granular layer. (Abstract in this study)