A vehicular battery mounting structure for mounting a battery pack above a floor panel while securing a simple structure of cooling passageways. This battery mounting structure is suitably adopted in a vehicle that is equipped with a first battery pack and a second battery pack that has stricter requirements regarding the temperature of the service environment. The first battery pack is mounted in an engine compartment. The second battery pack is disposed below a seat that is mounted on a floor panel as a seat for a vehicle occupant other than a driver. Specifically, a plurality of battery cells of the second battery pack are stacked in the longitudinal direction relative to the vehicle. Cooling passageways are provided between the battery cells, and conduct air supplied by a sirocco fan from a center side to an outer side in the direction of width of the vehicle.