Abstract: An electronic control apparatus in which each device unit is detachably mounted on an open front portion of a box-shaped main body by a press-and-clamp engaging mechanism. A nail plate is formed on both side faces of a terminal block. By engaging the nail plates with engaging holes formed in a unit main body, the terminal block can be attached to the unit main body and connected to a unit circuit board at the same time. A triangular color segment is attached to an indicator portion of each device unit for identification of its type. A palm-top type program setting unit is connected to a processing unit through lead wires. The inside circuitry of the setting unit is exposed by sliding its operation section.