To provide an underfloor structure of an automobile capable of discharging air in an engine compartment and efficiently reducing the air resistance generated by wheels.

In the underfloor structure of the automobile, an undercover 10 is arranged across a lower side of an engine compartment E and a lower side of a cabin R demarcated behind the engine compartment E, a first air passage 21 which is communicated with the engine compartment E and having air outlets 21a, 21a in a vicinity of and in front of front wheels FT1, FT2, a right second air passage 22a having an air outlet 23a in a vicinity of and in front of a right rear wheel RT1, and a left second air passage 22b having an air outlet 23b in a vicinity of and in front of a left rear wheel RT2 are formed between the engine compartment E and the cabin R. When a vehicle body S is traveling, air flowing into the engine compartment E is allowed to blow out of the air outlets 21a, 21a, 23a, 23b to reduce the air resistance generated on the front sides of the front wheels FT1, FT2 and the rear wheels RT1, RT2.