To provide a lower structure of a vehicle which fully exhibits a sensor function for detecting impact force to a vehicle side surface, without increasing weight.

A reinforcing member 7 is connected to an inner surface of a side sill inner panel 2 which together with a side body outer panel 6 forms a closed cross section, An upper surface 7J1 of an upper wall 7J of the reinforcing member 7 is connected to an inner surface 2J1 of an upper wall 2J of the side sill inner panel 2. A lower surface 7K1 of a lower wall 7K of the reinforcing member 7 is connected to an inner surface 2K1 of a lower wall 2K of the side sill inner panel 2. A sidewall 7S of the reinforcing member 7 is connected to an inner surface 2S1 of a sidewall 2S of the side sill inner panel 2. The sensor 3 for detecting impact force to the vehicle side surface and activating an airbag for side impact, the sidewall 2S of the side sill inner panel 2, and the sidewall 7S of the reinforcing member 7 are integrally fastened and fixed with a fastening member B.