This study focuses on validating a two-phase flow solver based on the modified Simple Coupled Level Set and Volume of Fluid method (Uchihashi et al. 2023) through viscous Kelvin-Helmholtz instability simulations.Our numerical simulation results are compared with the ones given by Funada and Joseph (2001) to provide reliable predictions of interface behavior under the influence of viscosity.The primary goal is to accurately assess the solver's ability to replicate theor. anal. of interface behaviors under various conditions.First, the wave between two fluids of identical d. is calculatedIn addition, the effect of surface tension is investigated.By comparing growth rates, numerical simulations obtain well-agreements with the anal. results on the effect of the fluid viscosity, the wave number, and the surface tension.Finally, fluid d. is changed to an air-water system.When relative velocity U is smaller than the criteria of relative velocity Uc given by anal. solutions, the wave is not broken.However, waves are splashed into droplets in the condition of U>Uc.This result agrees with the anal. by Funada and Joseph (2001).These findings provide a robust framework for applying the solver to more complex two-phase flow problems, supporting advancements in numerical simulations of fluid interfaces.