Among vehicles, there are automobiles of the hatchback type including a back door. In a vehicle body rear part of the vehicle of this type, a back door opening section having a round structure opened and closed by the back door is provided.
In the past, as a method of securing rigidity of the vehicle body rear part of such a vehicle, a high-rigidity section by a closed cross section of the round structure of the back door opening section and a rear side member section of a rear floor of a vehicle body basic structure are coupled and reinforced
However, in the structure of the vehicle body rear part in the past explained above, the high-rigidity section of the round structure of the back door opening section is provided in a position higher than the rear side member section of the rear floor of the vehicle body basic structure. The high-rigidity section of the round structure and the rear side member section of the vehicle body basic structure are laid out apart from each other in a vehicle up-down direction. In this case, it is necessary to add a separate bracket as disclosed in Japanese Patent Laid-Open No. 2006-218995 or substantially extend the high-rigidity section of the round structure of the back door opening section downward in the vehicle as shown in Japanese Examined Utility Model Publication No. H3-12621 to couple the high-rigidity section and the rear side member section. Therefore, in the structure of the vehicle-body rear part in the past, an increase in weight and an increase in costs are caused. Furthermore, it is likely that rigidity in the length direction is deteriorated because the high-rigidity section is vertically long.
In the structure of the vehicle body rear part in the past, when the rear side member section and a closed cross-sectional section in the vehicle width direction of the round structure of the back door opening section are coupled using a reinforcing member, there is a problem in that a load in the up-down direction from the round structure of the backdoor opening section cannot be efficiently transmitted to the rear side member.