A seat holding portion structure has a side member that extends substantially in a vehicle longitudinal direction. The side member has a kick portion whose front and rear end sides are bent and that joins a front portion and a rear portion of the side member that are offset in a vehicle vertical direction. A floor reinforcement is installed within the side member from an opening portion of a vehicle body floor. A rear end portion of an elongated seat holding member is inserted in the floor reinforcement. A connecting body having a closed cross-sectional structure is fixed to a rear end portion of the seat holding member and is disposed at a rear end side bent portion of the kick portion, and, together with the floor reinforcement, is mounted to the side member.