To facilitate program control modification and troubleshooting in sequence control system in spite of simple processing programs, sequence control processes are linked in a chain fashion. A unit process for activating an actuation member in both forward and reverse directions forms a ring so as to be activated in sequence cyclically via an execution condition recognizing switches, and these unit process rings are linked between an actuation start side activator for the present process and an actuation end side activator for the preceding process, both activated simultaneously whenever one of them is activated. Therefore, any process can be stopped at, restarted and reversed from any process steps in both the forward and reverse directions through the linked actuation start and end side activators, after the automatic operation has been switched to the manual operation or vice versa. Further, when a plurality of machines are controlled by a single sequencer, it is preferable to selectively connect a single operation board to a necessary robot connector to minimize the robot space and the cost.