A vehicular side airbag device that has a side airbag (4, 103) built into a seat and that, due to introduction of an inflator gas from an inflator, deploys and inflates in a direction from a vehicle rear towards a vehicle front, between an occupant and a vehicle side section, the vehicular side airbag device comprising: a top-bottom partitioning section (Ua) that is formed inside the side airbag (4, 103) in a vehicle front-rear direction, and that divides the interior of the side airbag (4, 103) into a lower bag region (Xa) and an upper bag region; and a front-rear partitioning section (Va) that is formed, inside the side airbag (4, 103), from the top-bottom partitioning section towards the top of the vehicle, and that divides the upper bag region into an upper front bag region (Ya) and an upper rear bag region, wherein the lower bag region and the upper rear bag region (Za) form a rapid chamber, and the upper front bag region forms a delay chamber; the inflator is disposed, on the vehicle-rear side inside the rapid chamber, from the upper rear bag region over the lower bag region; and a front-rear direction vent that allows the upper rear bag region of the rapid chamber to communicate with the delay chamber is provided in the front-rear partitioning section, wherein the side airbag (4, 103) comprises at least an exterior-side panel and an interior-side panel; and the front-rear partitioning section is formed through joining of the exterior-side panel and the interior-side panel, characterized in that the side airbag (4, 103) comprises a complementary exterior-side panel and a complementary interior-side panel that are respectively joined to the outer side of the exterior-side panel and the outer side of the interior-side panel; the complementary panels (106, 107) extend towards the vehicle front further than the front-rear partitioning section; and the delay chamber is formed on the vehicle-front side of the front-rear partitioning section, through joining of the complementary panels (106, 107) to each other. a top-bottom partitioning section (Ua) that is formed inside the side airbag (4, 103) in a vehicle front-rear direction, and that divides the interior of the side airbag (4, 103) into a lower bag region (Xa) and an upper bag region; and a front-rear partitioning section (Va) that is formed, inside the side airbag (4, 103), from the top-bottom partitioning section towards the top of the vehicle, and that divides the upper bag region into an upper front bag region (Ya) and an upper rear bag region, wherein the lower bag region and the upper rear bag region (Za) form a rapid chamber, and the upper front bag region forms a delay chamber; the inflator is disposed, on the vehicle-rear side inside the rapid chamber, from the upper rear bag region over the lower bag region; and a front-rear direction vent that allows the upper rear bag region of the rapid chamber to communicate with the delay chamber is provided in the front-rear partitioning section, wherein the side airbag (4, 103) comprises at least an exterior-side panel and an interior-side panel; and the front-rear partitioning section is formed through joining of the exterior-side panel and the interior-side panel, characterized in that the side airbag (4, 103) comprises a complementary exterior-side panel and a complementary interior-side panel that are respectively joined to the outer side of the exterior-side panel and the outer side of the interior-side panel; the complementary panels (106, 107) extend towards the vehicle front further than the front-rear partitioning section; and the delay chamber is formed on the vehicle-front side of the front-rear partitioning section, through joining of the complementary panels (106, 107) to each other.