To provide a control system and a communication cycle setting method enabling the side of slave equipment to determine priority of communication and to control communication cycles.

The control system includes: a communication cycle setting means 21 for setting priority of communication with master equipment on slave equipment 20 host side in accordance with a content of processing for each processing executed by connection equipment on a subordinate side of the slave equipment 20; a setting table storage means 22 for storing communication priority set for each processing; a communication cycle change determination means 23 for determining a change of communication priority when starting each processing; a communication cycle change notification means 24 for notifying the master equipment of the change; a connection equipment processing execution means 25 for controlling operation of the connection equipment; and a communication processing command transmission reception means 26 for receiving a communication processing command from the master equipment and responding to it. The master equipment communicates with the slave equipment 20 at a communication cycle corresponding to the communication priority notified by the slave equipment 20.