PURPOSE: To perform security not for a keyboard unit but for an entire system provided with a controller.

CONSTITUTION: When a program operating on a controller 7 instructs a security mode (key lock state) change, a processing part 9 transmits lamp data raising a flag instructing a security mode to a keyboard 1. A keyboard control part 5 of the keyboard 1 receiving the transmitted data starts the security mode corresponding to the processing of a microprogram, transmits key code data showing the execution of the security code to the controller 7 until canceling the security mode and operates just like apparently locking the keyboard 1. When the cancel of the security mode is instructed to the keyboard 1, regular key code data are transmitted to the controller 7. When the regular key code data are received, the controller 7 judges the end of the security mode and finishes a series of processing.