An on-vehicle control system comprising: a plurality of lower level control units (30-1, 30-2) each including a driver circuit capable of individually driving each of a plurality of loads (LSA to LSE); and an upper level control unit that is connected with each of the plurality of lower level control units (30-1, 30-2) via a predetermined communication line and controls the plurality of lower level control units (30-1, 30-2), wherein at least a first lower level control unit and a second lower level control unit are connected as the plurality of lower level control units (30-1, 30-2), the plurality of lower level control units (30-1, 30-2) each include a multiplex communication circuit; the upper level control unit includes a function allocation unit (22, 23) that allocates a first function to the first lower level control unit and allocates a second function to the second lower level control unit, and a master control unit that controls the first function and the second function by communication; each of the lower level control units (30-1, 30-2) includes a function execution unit (32, 33, 34) that controls each downstream load (LSA to SDE) according to the function allocated by the upper level control unit; characterized in that: each of the first lower level control unit and the second lower level control unit has a standard load connection unit to each of which the downstream load (LSA to LSE) is connected, and has an additional load connection unit to which an additional load (LOA, LOB) is connected; and the additional load connection unit is connected to the additional load (LOA, LOB) via an external driver. a plurality of lower level control units (30-1, 30-2) each including a driver circuit capable of individually driving each of a plurality of loads (LSA to LSE); and an upper level control unit that is connected with each of the plurality of lower level control units (30-1, 30-2) via a predetermined communication line and controls the plurality of lower level control units (30-1, 30-2), wherein at least a first lower level control unit and a second lower level control unit are connected as the plurality of lower level control units (30-1, 30-2), the plurality of lower level control units (30-1, 30-2) each include a multiplex communication circuit; the upper level control unit includes a function allocation unit (22, 23) that allocates a first function to the first lower level control unit and allocates a second function to the second lower level control unit, and a master control unit that controls the first function and the second function by communication; each of the lower level control units (30-1, 30-2) includes a function execution unit (32, 33, 34) that controls each downstream load (LSA to SDE) according to the function allocated by the upper level control unit; characterized in that: each of the first lower level control unit and the second lower level control unit has a standard load connection unit to each of which the downstream load (LSA to LSE) is connected, and has an additional load connection unit to which an additional load (LOA, LOB) is connected; and the additional load connection unit is connected to the additional load (LOA, LOB) via an external driver. each of the first lower level control unit and the second lower level control unit has a standard load connection unit to each of which the downstream load (LSA to LSE) is connected, and has an additional load connection unit to which an additional load (LOA, LOB) is connected; and the additional load connection unit is connected to the additional load (LOA, LOB) via an external driver.