A passenger protection apparatus, comprising: a vehicle seat having a seat cushion forming a seating face along with a seat back forming a backrest; and a side airbag apparatus housed in this seat, wherein: the seat back comprises a side support part which swells towards the front of the vehicle on the vehicle width direction side, a side frame having a frame side wall part which extends in the vehicle traveling direction when the horizontal cross section is seen from above is arranged inside the side support part, the side airbag apparatus comprises: an airbag for restraining a passenger when expanded and deployed, and an inflator for supplying expansion gas to the airbag, the airbag includes: a first chamber which houses the inflator and is deployed outside in the vehicle width direction of the frame side wall part; a second chamber which is deployed inside in the vehicle width direction of the frame side wall part and a gas guide arranged so as to surround the inflator inside the first chamber, the first chamber is deployed such that at least a portion thereof, as seen from the vehicle side, overlaps the frame side wall part, a first inner vent is formed at the boundary part between the gas guide and the first chamber, with gas introduced from the gas guide to the first chamber via this first inner vent, and a second inner vent is formed at the boundary part between the gas guide and the second chamber, with gas introduced from the gas guide to the second chamber via this second inner vent, characterized in that a third inner vent is formed at the boundary part between the first chamber and the second chamber, with gas introduced from the second chamber to the first chamber via this third inner vent. the seat back comprises a side support part which swells towards the front of the vehicle on the vehicle width direction side, a side frame having a frame side wall part which extends in the vehicle traveling direction when the horizontal cross section is seen from above is arranged inside the side support part, the side airbag apparatus comprises: an airbag for restraining a passenger when expanded and deployed, and an inflator for supplying expansion gas to the airbag, the airbag includes: a first chamber which houses the inflator and is deployed outside in the vehicle width direction of the frame side wall part; a second chamber which is deployed inside in the vehicle width direction of the frame side wall part and a gas guide arranged so as to surround the inflator inside the first chamber, the first chamber is deployed such that at least a portion thereof, as seen from the vehicle side, overlaps the frame side wall part, a first inner vent is formed at the boundary part between the gas guide and the first chamber, with gas introduced from the gas guide to the first chamber via this first inner vent, and a second inner vent is formed at the boundary part between the gas guide and the second chamber, with gas introduced from the gas guide to the second chamber via this second inner vent, characterized in that a third inner vent is formed at the boundary part between the first chamber and the second chamber, with gas introduced from the second chamber to the first chamber via this third inner vent.