The invention discloses a vehicular power supply device. A battery box 18 for accommodating a battery is mounted in a deviated manner to the left side of a vehicle body between right and left side frames 11 extending in the longitudinal direction of the vehicle body, and an intake duct 19 and an exhaust duct 20 of cooling air for cooling the battery are connected to an end of the battery box 18 on the right side of the vehicle body. Thus, the battery can be protected from shocks in the side impact by the right and left side frames 11, and at the same time, a space for arranging the intake duct 19 and the exhaust duct 20 can be ensured while ensuring the maximum capacity of the battery box 18. In addition, since right and left ends of the battery box 18 are connected to the right and left side frames 11, the battery box 18 can be firmly mounted on the vehicle body, and the rigidity of the right and left side frames 11 can be enhanced by the battery box 18.