A vehicular occupant protection system includes an air bag sensor; a base signal generating unit that generates a base signal for abnormality diagnosis for the air bag sensor; a diagnosis-signal generating unit that generates a diagnosis signal for the air bag sensor base on the base signal; an abnormality diagnosing unit that executes the abnormality diagnosis by using output signal that the air bag sensor outputs based on the diagnosis signal; and a condition-signal generating unit that generates a condition for a start of generating of the diagnosis signal. Here, the diagnosis-signal generating unit generates the diagnosis signal based on the base signal when the condition signal satisfies a given condition.