A drive force control system for a vehicle, comprising: an engine; a rotary machine; and a differential mechanism that is adapted to perform a differential rotation among the engine the rotary machine, and an output member (1R, 1L); the differential mechanism including: a first differential mechanism, having a first rotary element connected to the engine, a second rotary element connected to the rotary machine, and a third rotary element; and a second differential mechanism, having a fourth rotary element connected to the output member (1R, 1L), a fifth rotary element connected to the third rotary element, and a sixth rotary element; the drive force control system further comprising: a first engagement device for selectively connecting the first rotary element and the sixth rotary element; and a second engagement device for selectively connecting at least any two of the fourth rotary element, the fifth rotary element, and the sixth rotary element; wherein a normally stay clutch is used as at least one of the first engagement device and the second engagement device, and the normally stay clutch is adapted to be engaged upon reception of an engagement command signal and disengaged upon reception of a disengagement command signal, and to maintain a current engagement state when a signal transmission to the normally stay clutch is stopped, the differential mechanism being adapted to establish: a first operating mode in which a torque of the engine is delivered to the output member (1R, 1L) with a first predetermined ratio by engaging any one of the first engagement device and the second engagement device; and a second operating mode in which the torque of the engine is delivered to the output member (1R, 1L) with a second predetermined ratio that is smaller than the first predetermined ratio by engaging other one of the first engagement device and the second engagement device. an engine; a rotary machine; and a differential mechanism that is adapted to perform a differential rotation among the engine the rotary machine, and an output member (1R, 1L); the differential mechanism including: a first differential mechanism, having a first rotary element connected to the engine, a second rotary element connected to the rotary machine, and a third rotary element; and a second differential mechanism, having a fourth rotary element connected to the output member (1R, 1L), a fifth rotary element connected to the third rotary element, and a sixth rotary element; a first differential mechanism, having a first rotary element connected to the engine, a second rotary element connected to the rotary machine, and a third rotary element; and a second differential mechanism, having a fourth rotary element connected to the output member (1R, 1L), a fifth rotary element connected to the third rotary element, and a sixth rotary element; the drive force control system further comprising: a first engagement device for selectively connecting the first rotary element and the sixth rotary element; and a second engagement device for selectively connecting at least any two of the fourth rotary element, the fifth rotary element, and the sixth rotary element; a first engagement device for selectively connecting the first rotary element and the sixth rotary element; and a second engagement device for selectively connecting at least any two of the fourth rotary element, the fifth rotary element, and the sixth rotary element; wherein a normally stay clutch is used as at least one of the first engagement device and the second engagement device, and the normally stay clutch is adapted to be engaged upon reception of an engagement command signal and disengaged upon reception of a disengagement command signal, and to maintain a current engagement state when a signal transmission to the normally stay clutch is stopped, the differential mechanism being adapted to establish: a first operating mode in which a torque of the engine is delivered to the output member (1R, 1L) with a first predetermined ratio by engaging any one of the first engagement device and the second engagement device; and a second operating mode in which the torque of the engine is delivered to the output member (1R, 1L) with a second predetermined ratio that is smaller than the first predetermined ratio by engaging other one of the first engagement device and the second engagement device. a first operating mode in which a torque of the engine is delivered to the output member (1R, 1L) with a first predetermined ratio by engaging any one of the first engagement device and the second engagement device; and a second operating mode in which the torque of the engine is delivered to the output member (1R, 1L) with a second predetermined ratio that is smaller than the first predetermined ratio by engaging other one of the first engagement device and the second engagement device.