  We present a criterion for a shock wave existence in relativistic magnetic
hydrodynamics with an arbitrary (possibly non-convex) equation of state. The
criterion has the form of algebraic inequality that involves equation of state
of the fluid; it singles out the physical solutions and it can be easily
checked for any discontinuity satisfying concervation laws. The method of proof
uses introduction of small viscosity into the coupled set of equations of
motion of ideal relativistic fluid with infinite conductivity and Maxwell
equations.
