A fuel cell module , comprising: a cell stack apparatus comprising a cell stack including an array of a plurality of fuel cells, a manifold coupled to a fuel gas supply end of each of the fuel cells, for feeding a fuel gas to each of the fuel cells, and a reformer which reforms a raw fuel to produce a fuel gas; an oxygen-containing gas introduction plate having a discharge port for discharging oxygen-containing gas, the oxygen-containing gas introduction plate feeding an oxygen-containing gas to each of the fuel cells; a housing which houses the cell stack apparatus and the oxygen-containing gas introduction plate , the housing comprising a box having an open side and a lid which closes the open side of the box , a size of the open side of the box being greater than a size of the cell stack apparatus including the manifold and the reformer , to permit insertion of the cell stack apparatus lying on its side into the box from the opening of the box , wherein, as viewed from the open side, the oxygen-containing gas introduction plate lies closer to a bottom of the box than the cell stack apparatus ; a side heat-insulating material interposed between the cell stack apparatus and the housing so as to be located on a lateral side of the cell stack apparatus and extend along a direction of the array of the fuel cells of the cell stack apparatus ; and a cell stack-retaining heat-insulating material which retains the cell stack, characterised in that the cell stack-retaining heat-insulating material is fitted in a recess provided in a cell stack-side surface of the side heat-insulating material . a cell stack apparatus comprising a cell stack including an array of a plurality of fuel cells, a manifold coupled to a fuel gas supply end of each of the fuel cells, for feeding a fuel gas to each of the fuel cells, and a reformer which reforms a raw fuel to produce a fuel gas; an oxygen-containing gas introduction plate having a discharge port for discharging oxygen-containing gas, the oxygen-containing gas introduction plate feeding an oxygen-containing gas to each of the fuel cells; a housing which houses the cell stack apparatus and the oxygen-containing gas introduction plate , the housing comprising a box having an open side and a lid which closes the open side of the box , a size of the open side of the box being greater than a size of the cell stack apparatus including the manifold and the reformer , to permit insertion of the cell stack apparatus lying on its side into the box from the opening of the box , wherein, as viewed from the open side, the oxygen-containing gas introduction plate lies closer to a bottom of the box than the cell stack apparatus ; a side heat-insulating material interposed between the cell stack apparatus and the housing so as to be located on a lateral side of the cell stack apparatus and extend along a direction of the array of the fuel cells of the cell stack apparatus ; and a cell stack-retaining heat-insulating material which retains the cell stack, characterised in that the cell stack-retaining heat-insulating material is fitted in a recess provided in a cell stack-side surface of the side heat-insulating material . <EOS>