  Relativistic Sweet-Parker type magnetic reconnection is investigated by
relativistic resistive magnetohydrodynamic (RRMHD) simulations. As an initial
setting, we assume anti-parallel magnetic fields and a spatially uniform
resistivity. A perturbation imposed on the magnetic fields triggers magnetic
reconnection around a current sheet, and the plasma inflows into the
reconnection region. The inflows are then heated due to ohmic dissipation in
the diffusion region, and finally become relativistically hot outflows. The
outflows are not accelerated to ultra-relativistic speeds (i.e., Lorentz factor
~ 1), even when the magnetic energy dominates the thermal and rest mass
energies in the inflow region. Most of the magnetic energy in the inflow region
is converted into the thermal energy of the outflow during the reconnection
process. The energy conversion from magnetic to thermal energy in the diffusion
region results in an increase in the plasma inertia. This prevents the outflows
from being accelerated to ultra-relativistic speeds. We find that the
reconnection rate R obeys the scaling relation R S^{-0.5}, where S is the
Lundquist number. This feature is the same as that of non-relativistic
reconnection. Our results are consistent with the theoretical predictions of
Lyubarsky (2005) for Sweet-Parker type magnetic reconnection.
