[Object] To provide an airbag, an airbag apparatus, and an air-discharging method of the airbag which are able to reduce the pressure loss during inflation and deployment of the airbag, or is able to demonstrate the impact absorbing capability of the airbag apparatus efficiently depending on the physical structure of an occupant.[Solving Means] The airbag apparatus of the present invention includes a gas generator 1 configured to generate gas under predetermined conditions, an airbag 2 connected to the gas generator 1 and configured to be inflated and deployed, and a retainer 3 configured to store the airbag 2, and the airbag 2 includes a protrusion 4 which forms a portion protruding outward partly on the airbag 2, a vent hole 5 formed on the protrusion 4, a strap 6 connected at one end to the protrusion 4 and at the other end to the interior of the airbag 2, and an insertion hole 7 configured to guide the other end of the strap 6 into the interior of the airbag 2, and the vent hole 5 is arranged in an area where the protrusion 4 and the airbag 2 are overlapped with each other when the protrusion 4 is pulled by the strap 6.