To provide a mounting structure of a seat for a vehicle which can sufficiently secure a space for inserting a bolt inserted in the interior of a closed cross section, and which can easily mount a seat base on a floor panel with reliability and with high mounting strength even when the up-and-down length of the closed cross section in the vehicle width direction varies and partially becomes too small at the time of the installation of the seat base on a floor panel part of which the closed cross section is formed by the floor panel and a cross member.

The seat base 21 of the mounting structure of the seat for the vehicle is provided with a first extended part 30 which is extended to the side where the up-and-down length of the closed cross section 7 in the vehicle width direction becomes larger than the up-and-down length of the closed cross section 7 at the center of the seat base 21. Then a mounting part 40 is provided for mounting the first extended part 30 on the floor panel 4 by inserting a bolt 41 through the first extended part 30 from above and by screwing the bolt 41 to a nut 42 provided within the closed cross section 7.