PURPOSE: To effectively utilize a queuing time in the course of executing a command, by adding an executing display to a command which has become an object to be executed, so that the command can be updated to a stacker part in which the executing display is off.

CONSTITUTION: When a command is inputted from a keyboard KB, a command analytic means CMA provided in a command processing part CMP sends the command to a stack means STP, and the STP stores it in order to a stacker STK. Various messages in the course of executing the command are displayed on a display part DPL. Even in case of executing the command, when a command updating instruction of the STK is inputted from the KB, the STP discriminates by an executing display STKE of its command whether the command which has become an object to be updated is being executed or not. In case when it is being executed, a change disapproving message DPL is sent out to the DPL, and unless it is being executed, a designated command in the STK is updated.