  The Sausage radio relic is the arc-like radio structure in the cluster CIZA
J2242.8+5301, whose observed properties can be best understood by synchrotron
emission from relativistic electrons accelerated at a merger-driven shock.
However, there remain a few puzzles that cannot be explained by the shock
acceleration model with only in-situ injection. In particular, the Mach number
inferred from the observed radio spectral index, $M_{\rm radio}\approx 4.6$,
while the Mach number estimated from X-ray observations, $M_{\rm X-ray}\approx
2.7$. In an attempt to resolve such a discrepancy, here we consider the
re-acceleration model in which a shock of $M_s\approx 3$ sweeps through the
intracluster gas with a pre-existing population of relativistic electrons. We
find that observed brightness profiles at multi frequencies provide strong
constraints on the spectral shape of pre-existing electrons. The models with a
power-law momentum spectrum with the slope, $s\approx 4.1$, and the cutoff
Lorentz factor, $\gamma_{e,c}\approx 3-5\times 10^4$ can reproduce reasonably
well the observed spatial profiles of radio fluxes and integrated radio
spectrum of the Sausage relic. The possible origins of such relativistic
electrons in the intracluster medium remain to be investigated further.
