A serial communication system comprises: a master; a plurality of slaves; and a serial communication bus connecting the master and the plurality of slaves. The master is configured so as to perform peer-to-peer control, via the serial communication bus, of the plurality of slaves. Each of the plurality of slaves has: an I/O portion for controlling communication with the master; a control portion for controlling a driving portion of the slave; and a register portion. The register portion comprises a control program for the driving portion. The control program comprises a plurality of functions, and the register portion stores control information to which are allocated all or a portion of the plurality of functions corresponding to all or to a portion of the plurality of program steps. The master issues a command specifying the program step to each of the slaves, each of the slaves receiving the command via the serial communication bus, and each of the control programs of the plurality of slaves determines a program step specified by the command based on the received command, and the slaves simultaneously execute the function relevant to the relevant program step based on the control information.