  We construct a new Godunov type relativistic hydrodynamics code in Milne
coordinates, using a Riemann solver based on the two-shock approximation which
is stable under the existence of large shock waves. We check the correctness of
the numerical algorithm by comparing numerical calculations and analytical
solutions in various problems, such as shock tubes, expansion of matter into
the vacuum, the Landau-Khalatnikov solution, and propagation of fluctuations
around Bjorken flow and Gubser flow. We investigate the energy and momentum
conservation property of our code in a test problem of longitudinal
hydrodynamic expansion with an initial condition for high-energy heavy-ion
collisions. We also discuss numerical viscosity in the test problems of
expansion of matter into the vacuum and conservation properties. Furthermore,
we discuss how the numerical stability is affected by the source terms of
relativistic numerical hydrodynamics in Milne coordinates.
