Provided are an airbag, an airbag unit, and a vent hole open/close control method that facilitate control of opening and closing of a vent hole, and suppress an increase in weight and cost of the airbag unit vent hole.The airbag 1 includes a vent hole 4, an open/close device 5 configured to open and close the vent hole 4, and a stretch tether 6 stretchable in stages and connected to the open/close device 5 and an airbag component. The stretch tether 6 assumes, depending on an expansion and unfolding status of the airbag 1, a first stage including transition from a slack state to a tense state, a second stage including stretching to a predetermined length while maintaining the tense state, and a third stage including being slackened when the driver contacts the airbag 1. The open/close device 5 maintains the vent hole 4 in the open state in the first stage, maintains the vent hole 4 in the closed state in the second stage, and shifts the vent hole 4 to the open state in the third stage.