A fuel cell module (10, 20, 30), 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 (7A, 7B) 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 (8A, 8B, 8C, 8D) which retains the cell stack, characterised in that the cell stack-retaining heat-insulating material (8C, 8D) 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 (7A, 7B) 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 (8A, 8B, 8C, 8D) which retains the cell stack, characterised in that the cell stack-retaining heat-insulating material (8C, 8D) is fitted in a recess provided in a cell stack-side surface of the side heat-insulating material.