Vehicular lower structure

1. A vehicular lower structure including a battery pack mounted on a floor, wherein:

a front end of the battery pack is fixed to a floor cross member as a frame member extending in a vehicle width direction;

the floor cross member includes a cross member upper as an upper member and a cross member lower as a lower member;

the cross member upper includes an upward convex hat portion having a hat shape with an open lower section when seen from the vehicle width direction;

the cross member lower includes a downward convex hat portion having a hat shape with an open upper section when seen from the vehicle width direction;

a closed section structure is formed by fixing front flanges and rear flanges of the downward convex hat portion and the upward convex hat portion, respectively;

one of the cross member lower and the cross member upper includes an extension portion that is bent at a front end of the front flange, and extends downward to a front side to connect a front end of the extension portion to a floor panel; and

a reinforcement member is provided for the extension portion, the reinforcement member including:

a reinforcement flange that is bent adapted to a region from the front flanges of the cross member lower and the cross member upper to the extension portion, the reinforcement flange being joined and fixed to the front flanges and the extension portion; and

a rib plate inserted into an opening between the extension portion and the cross member lower while extending in a vehicle front-rear direction.

2. The vehicular lower structure according to claim 1 , wherein the plurality of reinforcement members are disposed for the floor cross member at equal intervals along the vehicle width direction.