An air bag system M1 is installed in a vehicle having front side doors 13 which are installed adjacent to the rear of front pillars 1 and which each have an outside rearview mirror 27. An air bag 42 includes a pillar covering portion 47 which is disposed to cover a front surface 1a of the front pillar 1 when it completes its inflation and a fitting cell portion 50 which is installed below the pillar covering portion 47 and which is disposed so that when it completes its inflation, a rear end side portion thereof fits in a space defined between the outside rearview mirror 27 and a lateral side of the vehicle on an upper surface side of the outside rearview mirror. The fitting cell portion 50 is connected to a lower edge side of the pillar covering portion 47 over substantially the whole area in a front-rear direction thereof.