Approximate boost of the relativistic two-body problem in instant formalism

Considering two spinless particles, a simple, approximate boost rule is derived that is sufficient to keep the mass invariant and to relate interactions, vertex functions, wave functions and t-matrices of the instant two-body problem in an arbitrary frame to those in the c.m. frame. The boost generator ${\bf K}$ that is used provides a solution to the Poincar\'{e} algebra except for the commutator between two components of ${\bf K}$, for which it provids an approximate solution.