In an occupant restraining device having a bag whose opposite ends are fixed to a seat pan, an inflated state of a gas chamber of the bag is specified. Both ends of a bag 12 in a left-right direction thereof are provided with stationary sections 12a and 12a that are fixed to a seat pan 8. The stationary sections 12a and 12a have therebetween a gas chamber 15 that is inflated as a result of gas entering from an inflator 13. Regarding the bag 12, a value of L2/L1, which is the ratio between a maximum width L2 of the gas chamber 15 in the left-right direction and a distance L1 between bolt through-holes 12b and 12b in the respective stationary sections 12a and 12a, is within a range of 0.6 to 0.9. Moreover, a value of L3/L2, which is the ratio between a maximum width L3 of the gas chamber 15 in a front-back direction and L2, is within a range of 0.4 to 0.85. Furthermore, a value of L1/L3, which is the ratio between L1 and L3, is less than or equal to 2.1.