To surely mount a power source device irrespective of a wheel base difference and to enhance safety of the power source device against external force such as an impact on a vehicle.

In the mounting structure of an on-vehicle power source device, a power source device 31 is mounted in a vehicle 10 including a pair of side members 11a and 11b arranged at a predetermined interval in a width direction by a plurality of cross members 11b and 11b. A feature of the configuration is that the power source device 31 is mounted between the pair of side members 11a and 11b. Preferably, the power source device 31 is mounted between the cross members 11b and 11b arranged in the front and back of a rear axle 14 hung on the pair of side members 11a and 11a.