An installation structure allowing installation of a capacitor as a battery unit in a hybrid vehicle, even in a vehicle not having a luggage compartment, is provided. A rear pillar cross member is mounted behind the rear seat in the cross section of the vehicle, and both ends of the rear pillar cross member are joined to inner panels of a wheel housing through a bracket and a reinforcing bracket plate. A top portion of the capacitor, which is a battery unit, is fixed to the rear pillar cross member, and the bottom portion of the capacitor is joined to the floor cross member, such that the capacitor is installed in a standing state. In addition, a setting structure for reliably setting a rear seat body on the floor of the vehicle is also provided.