cameraMouse_Klee.cs

public GameObject targetObj;
public float rotateSpeed = 2;
Vector3 offsetValue;

void Start() {
    offsetValue = targetObj.transform.position - transform.position;
}

void LateUpdate() {

    float horizontal = Input.GetAxis("Mouse X") * rotateSpeed;
    targetObj.transform.Rotate(0, horizontal, 0);

    float initAngle = targetObj.transform.eulerAngles.y;
    Quaternion rotation = Quaternion.Euler(0, initAngle, 0);

    transform.position = targetObj.transform.position - (rotation * offsetValue);

    transform.LookAt(targetObj.transform);

}