Down-time at switching of systems is reduced in a communication system that implements functional safety communication. Provided are a first preparation part 1222 that, when a functional safety communication initialization sequence is started between a slave and a control-system master, executes a first sequence group consisting of some sequences of the functional safety communication initialization sequence together with the slave; a second preparation part 1223 that executes a second sequence group being a sequence of the functional safety communication initialization sequence other than the first sequence group together with the slave when a failure of the control-system master is detected after execution of the first sequence group is completed; and a control communication control part 110 that starts control communication with the slave after execution of the second sequence group is completed.