  We construct a new relativistic viscous hydrodynamics code optimized in the
Milne coordinates. We split the conservation equations into an ideal part and a
viscous part, using the Strang spitting method. In the code a Riemann solver
based on the two-shock approximation is utilized for the ideal part and the
Piecewise Exact Solution (PES) method is applied for the viscous part. We check
the validity of our numerical calculations by comparing analytical solutions,
the viscous Bjorken's flow and the Israel-Stewart theory in Gubser flow regime.
Using the code, we discuss possible development of the Kelvin-Helmholtz
instability in high-energy heavy-ion collisions.
