To provide a car cabin structure with which the intended use of the intra-cabin space can be changed simply according to the necessity by introducing such an arrangement that a box to accommodate infant, pet animal, etc., can be simply and certainly secured to the inside of the space which has got rid of the seat.

A car cabin structure is arranged so that a seat is installed removably on the car body floor 5 side with the aid of mounting/removing mechanisms 8, 9, 11, 12. A unit 6 furnished at the bottom surface with a mounting/removing mechanism having interchangeability with those 8, 9, 11, 12 is made capable of being installed when necessary on the car body floor 5 side in lieu of the seat.