[Object] To provide an airbag unit smaller in size and lighter in weight, and capable of being rapidly expanded, and a method of folding an airbag.[Solution] An airbag unit 20, configured to cover at least a part of a windshield 6 and a front pillar 2 outside of a vehicle 1, includes an airbag 10 formed in a U-shape including a bottom portion and a pair of projecting portions respectively projecting from the bottom portion, a gas generator 22 that ejects gas into the airbag 10, a lower cover unit 21 that accommodates therein the unexpanded airbag 10 and the gas generator 22, and an upper cover unit 24 that covers the lower cover unit 21 containing therein the airbag 10. The airbag 10 is expanded by the gas ejected from the gas generator 22 through between a hood 4 the windshield 6 of the vehicle 1.