To improve the transmission efficiency of communication and to accelerate transmission by making a host device output an ID corresponding to a command instead of the command and making a receiving subordinate device read the command corresponding to the ID and executing the command.

This system is provided with the host device 10 for issuing the command 31 and the subordinate device 20 for executing the command 31. The host device 10 is provided with an ID storage part 12 for storing a prescribed command 31 and ID 34 making them correspond to each other and a command issuing part 11 for outputting the ID 34 corresponding to the pertinent command 31 instead of the command 31 in the case that the ID 34 corresponding to the command 31 to be issued is stored in the ID storage part 12. In the meantime, the subordinate device 20 is provided with a command storage part 33 for storing a prescribed ID 34 and command 31 making them correspond to each other, an ID processing part 21 for reading the command 31 corresponding to the ID 34 outputted from the host device 10 form the command storage part 22 and a command execution part 24 for executing the command 31 read by the ID processing part 21.