To improve the rigidity of a side sill without bringing about an increase in a weight or costs regarding the connected part structure of a pillar and the side sill.

A pillar 1 having a seat belt retractor 3 for taking up a seat belt 4 in an inner part lower direction is arranged roughly in a vertical direction, a side sill 2 connected to the lower end of the pillar 1 is arranged in a car longitudinal direction, a step part 6 for preventing the interference of the seat belt retractor 3 is formed on the pillar 1 connecting part of the side sill 2 and a recessed groove 7 for securing the rigidity of the side sill 2 in the car longitudinal direction is provided in the step part 6 along the car longitudinal direction.