We developed a new numerical model which can simulate the thermal history and metal-silicate separation of a growing Mars. In this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating, crater excavation, and isostatic rebound. A metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature. Sinking of the metal blobs is traced assuming Stokes' velocity. Their coalescence during sinking is treated by a Monte Carlo approach. A series of simulations is carried out assuming that Mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process. Our numerical results show that (1) no global magma ocean is formed during accretion, (2) metal-silicate separation takes place without global scale melting, and (3) instead of a metallic core, a metal-rich layer is formed at the late stage of accretion.