A lateral surface member structure (100) of a vehicle body (1) includes a tubular body (110) extending in a front-rear direction of the vehicle body (1) and an impact absorbing member (120) disposed inside the tubular body (110). The impact absorbing member (120) includes a web (121) extending along the front-rear direction and flat in a vehicle width direction, a vehicle outer flange (122) joined to a vehicle outer end portion of the web (121) and extending along the front-rear direction, and a vehicle inner flange (123) joined to a vehicle inner end portion of the web (121) and extending along the front-rear direction. The vehicle outer flange (122) and the vehicle inner flange (123) each include a rib (122R, 123R) disposed so as to sandwich the web (121) from above and below and extending along the front-rear direction.