Automotive side airbag and method of operating same

An automotive side airbag comprising:

• an inflator ejecting gas when operating;
• an air cushion expanded and unfolded forward from a side of a passenger by the gas ejected from the inflator;
• a diffuser disposed in the air cushion, supplying the gas ejected from the inflator to the air cushion, and having a discharge hole formed across a direction in which the air cushion is unfolded by the supplied gas;
• and
• a separator dividing an internal space of the air cushion into an upper chamber and a lower chamber, wherein the diffuser communicates with both of the upper chamber and the lower chamber, and wherein the separator is coupled to an outer side of the diffuser under the discharge hole.