A solid oxide fuel cell device, comprising: a cell stack including multiple fuel cell units in combination; a reformer for reforming fuel gas supplied to the fuel cell units; a combustion portion for heating the reformer and the cell stack using exhaust gas produced by combusting surplus fuel gas or the reformed fuel gas passed through the fuel cell units; a temperature detector (142, 148) for detecting the temperature of the cell stack and the temperature of the reformer; a module housing chamber for housing the cell stack and the reformer; a control means for starting up the fuel cell device; wherein the control means is constituted so that in the fuel cell device startup process, supply flow rates of fuel gas, oxidant gas, and steam supplied to the reformer are controlled based on the cell stack temperature and the reformer temperature, and after a partial oxidation reforming process, an autothermal reforming process, and a steam reforming process are performed in sequence by the reformer in a fuel gas reformer reaction process, a transition is made to an electrical power generating process, and when the cell stack temperature and the reformer temperature reach transition temperatures set to each of the processes in the fuel gas reformer reaction process, a transition is made to the next process; characterized in that the fuel cell device comprises: a heat storage means disposed around the module housing chamber; a determining means for determining whether a temperature rise assist state occurs in which a temperature rise in the reformer and/or cell stack is assisted by the heat amount stored by the heat storage means during startup of the fuel cell device; wherein when the determining means determines an occurrence of the temperature rise assist state, the control means performs a control to cause a transition to the electrical power generating process in a state in which the flow rate of fuel gas supplied is less than when the determining means does not determine an occurrence of the temperature rise assist state. a cell stack including multiple fuel cell units in combination; a reformer for reforming fuel gas supplied to the fuel cell units; a combustion portion for heating the reformer and the cell stack using exhaust gas produced by combusting surplus fuel gas or the reformed fuel gas passed through the fuel cell units; a temperature detector (142, 148) for detecting the temperature of the cell stack and the temperature of the reformer; a module housing chamber for housing the cell stack and the reformer; a control means for starting up the fuel cell device; wherein the control means is constituted so that in the fuel cell device startup process, supply flow rates of fuel gas, oxidant gas, and steam supplied to the reformer are controlled based on the cell stack temperature and the reformer temperature, and after a partial oxidation reforming process, an autothermal reforming process, and a steam reforming process are performed in sequence by the reformer in a fuel gas reformer reaction process, a transition is made to an electrical power generating process, and when the cell stack temperature and the reformer temperature reach transition temperatures set to each of the processes in the fuel gas reformer reaction process, a transition is made to the next process; characterized in that the fuel cell device comprises: a heat storage means disposed around the module housing chamber; a determining means for determining whether a temperature rise assist state occurs in which a temperature rise in the reformer and/or cell stack is assisted by the heat amount stored by the heat storage means during startup of the fuel cell device; wherein when the determining means determines an occurrence of the temperature rise assist state, the control means performs a control to cause a transition to the electrical power generating process in a state in which the flow rate of fuel gas supplied is less than when the determining means does not determine an occurrence of the temperature rise assist state.