To improve productivity of a vehicle body by facilitating formation of a sidewall in a sidewall reinforcing structure of the vehicle body provided with a guide rail supported to a lower part of the sidewall of the vehicle body to guide a sliding door for opening and closing a door opening formed in the sidewall of the vehicle body.

The vehicle includes: a box-shaped bracket 43 extending in a front-back direction of the vehicle body 2 and being fitted in a rocker 17 configuring a lower part of the sidewall 6 of the vehicle body 2; and the guide rail 38 fitted in the bracket 43 to guide the sliding door 34 to open and close the door opening 15 formed in the sidewall 6. The part on an inner side of the vehicle body 2 at a front end of the bracket 43 is an extending part extending toward the inner side of the vehicle body 2. A through hole 46 is formed in an inner panel 12 of the rocker 17. The extending part 43a of the bracket 43 is formed to pass the through hole 46 and to extend to the inner side of the vehicle body 2. The extending part 43a of the bracket 43 is joined to a top plate 28 of a cross member 24.