A fuel cell apparatus, comprising: a stack to which an anode gas and a cathode gas are supplied and which generates electricity by the supplied anode gas and the supplied cathode gas; a heat insulating layer covering an outside of the stack; a cathode gas flow passage through which the cathode gas is supplied to the stack; an exhaust gas flow passage through which an exhaust gas generated by a power generating operation of the stack is discharged to an outside of the fuel cell apparatus; a housing facing the stack in a state where the heat insulating layer is arranged between the housing and the stack, the housing accommodating the stack, the heat insulating layer, the cathode gas flow passage, and the exhaust gas flow passage and including a side wall (200s, 212, 222, 232) and a bottom wall (200b, 213, 223, 233), wherein at least one of the side wall (200s, 212, 222, 232) and the bottom wall (200b, 213, 223, 233) in a cross section taken along a vertical direction of the housing or a horizontal direction of the housing is formed to be curved outwardly so that an intermediate area (200c, 212c, 222c, 232c, 200bc) at at least one of the side wall (200s, 212, 222, 232) and the bottom wall (200b, 213, 223, 233) is positioned further away from the stack than both ends of at least one of the side wall (200s, 212, 222, 232) and the bottom wall (200b, 213, 223, 233), characterized in that in a case where a protruding amount of the intermediate area (200c, 200bc) of the side wall or the bottom wall protruding outwardly from an imaginary line (P1, P2) in the cross section is defined as a maximum curvature protruding amount [delta] and a length of the side wall in the vertical direction or a length of the bottom wall in the horizontal direction is defined as a length (HA1, HA2), a ratio of the maximum curvature protruding amount [delta] relative to the length (HA1, HA2) is within a range from 1 × 10 -5 to 1 × 10 -1, the imaginary line (P1, P2) imaginarily connecting an inner wall surface at a first area of the side wall and an inner wall surface at a second area of the side wall along the vertical direction or imaginary connecting an inner wall surface at a first area of the bottom wall and an inner wall surface at a second area of the bottom wall along the horizontal direction. a stack to which an anode gas and a cathode gas are supplied and which generates electricity by the supplied anode gas and the supplied cathode gas; a heat insulating layer covering an outside of the stack; a cathode gas flow passage through which the cathode gas is supplied to the stack; an exhaust gas flow passage through which an exhaust gas generated by a power generating operation of the stack is discharged to an outside of the fuel cell apparatus; a housing facing the stack in a state where the heat insulating layer is arranged between the housing and the stack, the housing accommodating the stack, the heat insulating layer, the cathode gas flow passage, and the exhaust gas flow passage and including a side wall (200s, 212, 222, 232) and a bottom wall (200b, 213, 223, 233), wherein at least one of the side wall (200s, 212, 222, 232) and the bottom wall (200b, 213, 223, 233) in a cross section taken along a vertical direction of the housing or a horizontal direction of the housing is formed to be curved outwardly so that an intermediate area (200c, 212c, 222c, 232c, 200bc) at at least one of the side wall (200s, 212, 222, 232) and the bottom wall (200b, 213, 223, 233) is positioned further away from the stack than both ends of at least one of the side wall (200s, 212, 222, 232) and the bottom wall (200b, 213, 223, 233), characterized in that in a case where a protruding amount of the intermediate area (200c, 200bc) of the side wall or the bottom wall protruding outwardly from an imaginary line (P1, P2) in the cross section is defined as a maximum curvature protruding amount [delta] and a length of the side wall in the vertical direction or a length of the bottom wall in the horizontal direction is defined as a length (HA1, HA2), a ratio of the maximum curvature protruding amount [delta] relative to the length (HA1, HA2) is within a range from 1 × 10 -5 to 1 × 10 -1, the imaginary line (P1, P2) imaginarily connecting an inner wall surface at a first area of the side wall and an inner wall surface at a second area of the side wall along the vertical direction or imaginary connecting an inner wall surface at a first area of the bottom wall and an inner wall surface at a second area of the bottom wall along the horizontal direction.