A battery mounting structure for maintaining a sufficient passenger compartment in vehicles is provided. The battery mounting structure comprises a pair of longitudinal side sills (11, 12), and a battery pack (7) disposed between the side sills (11, 12). A plurality of battery modules (9) are juxtaposed in a casing (8) of the battery pack (7). A linear depression (39) protrudes downwardly from a reinforcement member (15) situated underneath a floor panel (17) supported by the side sills (11, 12), and the battery pack (7) has a linear depression (38) hold the linear depression (39) therein.