To provide a comparator for an underfloor structure, which allows an easy grasp of a difference between underfloor ventilation structures and can give an impression that the ventilation structure using a sill spacer is excellent.

In an underfloor structure simulator 2 using the sill spacer, the inside of a housing 20 is partitioned into four compartments (a), (b), (c) and (d); plate pieces 22 are provided on a top surface of the housing 20 with proper gaps among them; and a transparent plate 23 is provided on the transparent plate 23. In an underfloor structure simulator 3 using a ventilating opening, the inside of a housing 30 is partitioned into four compartments (a), (b), (c) and (d); an opening 32 is provided in each of the compartments (a), (b), (c) and (d) of the housing 30; and transparent plates 33 are provided on a top surface of the housing 30. An intake system 5 makes air taken into the two compartments (a) and (b) of the simulators 2 and 3; and a flag 4, which is swung by the air taken in by the intake device (5), is provided in each of the compartments (a), (b), (c) and (d). The comparator 1 for the underfloor structure allows a difference in ventilation performance, brought about by a difference between the respective underfloor structures, to be visually identified by comprising swinging manners of the flags 4 of both the simulators 2 and 3.