PURPOSE: To lighten the burden imposed on users of a system by sending an error response after its production when commands received by a reader/writer from a terminal are decided to be a multi-command.

CONSTITUTION: A command 1 is given to an IC card 11 through a reader/writer R/W13 from a terminal 15, whereby a microprocessor 25 in the IC card 11 performs prescribed processing, records it in a PROM31 and transmits a response to the terminal 15 through the R/W13. If a command 2 is given to the R/W13 from the terminal 15 during this transmission, the R/W13 decides it to be a multi-command when the response of the command 1 is not received from the IC card 11, generates error codes with respect to the command 2 and transmits them to the terminal 15. It displays automatically an error message, shows that the command 2 is invalid and deals with a normal response arriving subsequently as the command 1.