This report deals with a heat conduction problem of flat plate like a large-scale solar array panel used in space. It is convenient to use two-dimensional in-plane heat conduction model when we perform buckling analysis and out-of-plane deformation analysis of flat plate structure under transient temperature variation. But heat transfer on plate surface is much greater than heat transfer through its edge in two-dimensional model, so the analysis which is based on the two-dimensional heat conduction equation is inadequate. Under the assumption that the temperature distribution in the thickness direction of the plate is linear, we derive an effective heat conduction equation system considering considered the heat transfer between the upper and lower surfaces of the plate.