Airbag and airbag system

To provide an airbag and an airbag system in which the left airbag section and the right airbag section inflate sufficiently early even when the capacity of the inflator is relatively low and do not interfere with an A-pillar and so forth. An airbag 10 includes a left airbag section 12, a left airbag section 14, a communicating part 16 that communicates the base ends of the right airbag section 12 and the left airbag section 14 with each other, and tethers 70 that join the left and right sides of the right airbag section 12 and the left and right sides of the left airbag section 14. The tether 70 of the bag 14 adjacent to the car body is joined with the upper part of the car-body side panel 28 (Fig. 1).