To enable an adult occupant to seat on the seating surface of an in-vehicle seat without removing a child seat to improve convenience.

In the child seat mounting structure of this vehicle, the in-vehicle seat 4 provided in the cabin consists of a seat cushion 11 having a seating surface 11a approximately parallel to the cabin floor 1, and a seat back 12 which pivotably mounted with a pivot part 14 provided on the rear end part of the seat cushion 11 in the fore and aft direction as a center. The seat back 12 can be laid and superposed on the seating surface 11a of the seat cushion. A fixing base member to fix the child seat 40 is provided on the back surface 12a of the seat back.