This paper proposes a new mathematical model for heat transfer in water-steam flow under the boiling condition in a porous medium and its comparison with the experimental data already reported by the authors. The conclusions are as follows:(1) The calculation obtained from the new mathematical model almost agrees with the experimental data of the steam ratio.(2) The apparent thermal conductivity defined in the mathematical model is an important factor in estimation of heat transfer of the water-steam flow accompanied by boiling in a porous medium.(3) The apparent thermal conductivity decreases remarkably in the region of less than 0.2 of water saturation.