A vehicle mounting structure for a fuel cell system having a fuel cell unit and a fuel cell auxiliary unit, comprising: a flat floor; a pair of floor frames (105, 106) provided on sides of a vehicle body along a longitudinal direction of a vehicle, and joined to an underside of the flat floor; cross members (104, 141, 142, 143) provided in a width direction of the vehicle and connected to the pair of floor frames (105, 106), and joined to the underside of the flat floor; a first region defined by the pair of floor frames (105, 106) and a first pair of the cross members (141, 142), the fuel cell unit being disposed so as to be close to the flat floor in the first region; a second region defined by the pair of floor frames (105, 106) and a second pair of the cross members (104, 141) so as to be adjacent to the first region in the longitudinal direction of the vehicle, the fuel cell auxiliary unit being disposed so as to be close to the flat floor in the second region; characterized by a first pair of brackets sandwiching the fuel cell unit in the longitudinal direction of the vehicle, attached to bottom surfaces of the cross members (141, 142), and attaching the fuel cell unit to the first pair of the cross members (141, 142); a second pair of brackets sandwiching the fuel cell unit in the width direction of the vehicle, attached to floor frames (105, 106), and attaching the fuel cell unit to the pair of floor frames (105, 106); and an under cover covering a bottom portion of the fuel cell unit, wherein each of the first pair of brackets and the second pair of brackets is configured to be a flange (152a, 152b) extending from the under cover. a flat floor; a pair of floor frames (105, 106) provided on sides of a vehicle body along a longitudinal direction of a vehicle, and joined to an underside of the flat floor; cross members (104, 141, 142, 143) provided in a width direction of the vehicle and connected to the pair of floor frames (105, 106), and joined to the underside of the flat floor; a first region defined by the pair of floor frames (105, 106) and a first pair of the cross members (141, 142), the fuel cell unit being disposed so as to be close to the flat floor in the first region; a second region defined by the pair of floor frames (105, 106) and a second pair of the cross members (104, 141) so as to be adjacent to the first region in the longitudinal direction of the vehicle, the fuel cell auxiliary unit being disposed so as to be close to the flat floor in the second region; characterized by a first pair of brackets sandwiching the fuel cell unit in the longitudinal direction of the vehicle, attached to bottom surfaces of the cross members (141, 142), and attaching the fuel cell unit to the first pair of the cross members (141, 142); a second pair of brackets sandwiching the fuel cell unit in the width direction of the vehicle, attached to floor frames (105, 106), and attaching the fuel cell unit to the pair of floor frames (105, 106); and an under cover covering a bottom portion of the fuel cell unit, wherein each of the first pair of brackets and the second pair of brackets is configured to be a flange (152a, 152b) extending from the under cover.