In recent years, microfluidic devices have been frequently used to handle highly concentrated solutions. In these cases, the influence of the viscosity of the solution on the flow and diffusion in the microfluidic device cannot be neglected. In this study, the hydrodynamic and diffusion behaviors of two liquids having different viscosities in contact in a double-Y-type microfluidic device were investigated. The relationship between the concentration and the flow rate at the outlet of the microfluidic device and the ratio of viscosities of the solution and the solvent was determined. Computational fluid dynamics (CFD) was used to predict the flow in the microfluidic device. The diffusion of a Co(2+)-DEHPA complex in the microfluidic device was also investigated. When two fluids with different viscosities were pumped into the channel at the same flow rate, the high-viscosity fluid flowed to the low-viscosity side. The cross section of the low-viscosity fluid became narrow, the velocity increased, and the interface between the two fluids moved to the low-viscosity fluid side. When the viscosity ratio was large, the interface shift and velocity difference were large. The numerical simulation performed reproduced the influence of the viscosity ratio on the velocity and concentration distribution. The movement of the interface could be prevented by increasing the relative flow rate the of low-viscosity fluid, and this was also reproduced by the numerical simulation.