A system comprising a control device, a sensor and a control target, wherein the control device comprises: an electronic control unit that processes an electric signal from the sensor detecting a physical value, and outputs an electric signal to control the control target; and a box-shaped housing main body that is configured by a conductive material and that accommodates the electronic control unit, a mounting stay that supports the housing main body, that is configured by a conductive material and that is electrically connectable to a vehicle body at a vehicle body connection part, wherein the electronic control unit includes an internal ground electrically connected to the housing main body and a power circuit that supplies electric power for driving the sensor, wherein the mounting stay has a first mounting part that extends along a first lateral part of the housing main body and a second mounting part that extends from one end of the first mounting part along a second lateral part of the housing main body, wherein the control device is mountable on the vehicle body such that the vehicle body connection part is connected to an end of the second mounting part which is distal from the one end of the first mounting part, wherein the housing main body is electrically connected to the first mounting part and the second mounting part at the first lateral part and the second lateral part respectively, wherein the internal ground is divided into a high-power internal ground provided inside of the housing main body on a side of the second mounting part so as to be near the vehicle body connection part, and a low-power internal ground provided inside of the housing main body on a side of the first mounting part such that the high-power internal ground is electrically connected via the housing main body to the second mounting part and the low-power internal ground is electrically connected via the housing main body to the first mounting part so that the electrical distance from the vehicle-body connection part to the low-power internal ground is farther than the electrical distance from the vehicle-body connection part to the high-power internal ground, wherein a ground of the control target is connected to the high-power internal ground, wherein a ground of the power circuit is connected to the low-power internal ground, and wherein a ground of the sensor is connected outside of the housing main body to the other end of the first mounting part. an electronic control unit that processes an electric signal from the sensor detecting a physical value, and outputs an electric signal to control the control target; and a box-shaped housing main body that is configured by a conductive material and that accommodates the electronic control unit, a mounting stay that supports the housing main body, that is configured by a conductive material and that is electrically connectable to a vehicle body at a vehicle body connection part, wherein the electronic control unit includes an internal ground electrically connected to the housing main body and a power circuit that supplies electric power for driving the sensor, wherein the mounting stay has a first mounting part that extends along a first lateral part of the housing main body and a second mounting part that extends from one end of the first mounting part along a second lateral part of the housing main body, wherein the control device is mountable on the vehicle body such that the vehicle body connection part is connected to an end of the second mounting part which is distal from the one end of the first mounting part, wherein the housing main body is electrically connected to the first mounting part and the second mounting part at the first lateral part and the second lateral part respectively, wherein the internal ground is divided into a high-power internal ground provided inside of the housing main body on a side of the second mounting part so as to be near the vehicle body connection part, and a low-power internal ground provided inside of the housing main body on a side of the first mounting part such that the high-power internal ground is electrically connected via the housing main body to the second mounting part and the low-power internal ground is electrically connected via the housing main body to the first mounting part so that the electrical distance from the vehicle-body connection part to the low-power internal ground is farther than the electrical distance from the vehicle-body connection part to the high-power internal ground, wherein a ground of the control target is connected to the high-power internal ground, wherein a ground of the power circuit is connected to the low-power internal ground, and wherein a ground of the sensor is connected outside of the housing main body to the other end of the first mounting part.