A front airbag including a sub-string and a vent. The front airbag attenuates impact in order to provide protection to a child or an occupant in an abnormal position on a passenger seat. Upon a vehicle collision, the front airbag discharges internal air through a vent provided at the lateral surface of the front airbag, if the front airbag is not inflated at the front, but is instead laterally inflated. The front airbag includes a main tether provided at a central portion of an airbag cushion, a vent selectively discharging air to an outside from the front airbag, and a sub-string, connected to and between the main tether, to control opening/closing of the vent part.