Airbag device and airbag folding method

Provided are an airbag device and an airbag folding method capable of enhancing deployment performance of an airbag without using a separate component. An airbag (2) includes a restraint portion (21) to be located in front of an occupant upon inflation and deployment, a base portion (22) connected to an inflator (3), a top portion (23) to be located on a windshield (W) side upon inflation and deployment, and a bottom portion (24) to be located between the occupant and an instrument panel (I) upon inflation and deployment. The restraint portion (21) is folded in a belt shape with a contact surface (21a) of the restraint portion (21) for contact with the occupant facing up to form a flap portion (25), and the flap portion (25) is folded to move while turning toward the occupant upon inflation and deployment of the airbag (2).