A vehicle side airbag device with a decreased load on the chest of passenger and an increased initial restraining performance resulting from a side airbag. In the side airbag device, a vent hole portion is formed from the stitching portion between a main panel and a side face panel being interrupted at a portion thereof. Flaps provided to the main panel and the side face panel are inserted from the vent hole portion into the chest chamber. The flaps close off the vent hole portion by means of the internal pressure of the chest chamber until the side airbag inflated and deployed between an occupant and a door trim. When the internal pressure of the chest chamber rise by means of chest chamber being compressed between the occupant and the door trim, the flaps are pushed out of an airbag main body portion, and the vent hole portion is opened.