A roll over detector for a vehicle is disclosed, which includes a roll over phenomenon detecting circuit that detects a roll over phenomenon of the vehicle based on a relationship between at least a type of physical quantity representative of a roll state of the vehicle and a predetermined threshold; a reacting roll phenomenon detecting circuit that detects a reacting roll phenomenon of the vehicle based on a predetermined criterion, which reacting roll phenomenon occurs in reaction to the roll phenomenon in an opposite direction; and a threshold changing circuit that changes the predetermined threshold from a first threshold to a second threshold so that the roll over phenomenon becomes difficult to be detected by the roll over phenomenon detecting circuit if the reacting roll phenomenon is detected by the reacting roll phenomenon detecting circuit.