A gas generating composition exhibiting good slag forming ability is provided. The gas generating composition includes a fuel ranging from 10 to 60 % by mass, an oxidizing agent ranging from 10 to 70 % by mass, and a cooling agent (other than iron oxide) ranging from 1 to 20 % by mass. The cooling agent has a volume mean diameter (D50) of 10 to 70 µm and a volume mean diameter at cumulative of 10% (D10) of equal to or greater than 5 µm. The cooling agent acts to decrease the combustion temperature and improve slag forming ability.