A vehicular passenger restraint device including a seatbelt capable of restraining a passenger seated in a rear seat that includes a seat cushion and a seat back, the vehicular passenger restraint device comprising: a belt guide disposed on an outer side, in a vehicle width direction, of an upper portion of the seat back the belt guide holding the seatbelt inserted thereinto in a longitudinal direction of a vehicle body, the belt guide including a guide portion supporting the seatbelt from a lower side of the vehicle body and guiding the seatbelt, and an inner longitudinal surface portion provided on the inner side, in the vehicle width direction, of the guide portion and extending upward of the vehicle body, characterized in that the inner longitudinal surface portion being configured to be tiltable toward the inner side in the vehicle width direction by a load directed toward the inner side in the vehicle width direction, applied from the seatbelt at the time of collision, wherein a lock release switch for releasing lock keeping the seat back in an upright state is disposed on the inner side, in the vehicle width direction, of the upper portion of the seat back relative to the belt guide; and a regulation member regulating the inner longitudinal surface portion between the belt guide and the lock release switch not to be inclined toward the inner side in the vehicle width direction by a predetermined angle or more. that includes a seat cushion and a seat back, the vehicular passenger restraint device comprising: a belt guide disposed on an outer side, in a vehicle width direction, of an upper portion of the seat back the belt guide holding the seatbelt inserted thereinto in a longitudinal direction of a vehicle body, the belt guide including a guide portion supporting the seatbelt from a lower side of the vehicle body and guiding the seatbelt, and an inner longitudinal surface portion provided on the inner side, in the vehicle width direction, of the guide portion and extending upward of the vehicle body, characterized in that the inner longitudinal surface portion being configured to be tiltable toward the inner side in the vehicle width direction by a load directed toward the inner side in the vehicle width direction, applied from the seatbelt at the time of collision, wherein a lock release switch for releasing lock keeping the seat back in an upright state is disposed on the inner side, in the vehicle width direction, of the upper portion of the seat back relative to the belt guide; and a regulation member regulating the inner longitudinal surface portion between the belt guide and the lock release switch not to be inclined toward the inner side in the vehicle width direction by a predetermined angle or more. a belt guide disposed on an outer side, in a vehicle width direction, of an upper portion of the seat back the belt guide holding the seatbelt inserted thereinto in a longitudinal direction of a vehicle body, the belt guide including a guide portion supporting the seatbelt from a lower side of the vehicle body and guiding the seatbelt, and an inner longitudinal surface portion provided on the inner side, in the vehicle width direction, of the guide portion and extending upward of the vehicle body, characterized in that the inner longitudinal surface portion being configured to be tiltable toward the inner side in the vehicle width direction by a load directed toward the inner side in the vehicle width direction, applied from the seatbelt at the time of collision, wherein a lock release switch for releasing lock keeping the seat back in an upright state is disposed on the inner side, in the vehicle width direction, of the upper portion of the seat back relative to the belt guide; and a regulation member regulating the inner longitudinal surface portion between the belt guide and the lock release switch not to be inclined toward the inner side in the vehicle width direction by a predetermined angle or more.