PURPOSE: To simply add a command or command syntax requested by a user by previously recording a key ward for the command or command syntax in a system table.

CONSTITUTION: If a user registers a key ward SQRT in a system together with a source program when no function including a square root (SQRT) is provided to a control system, the key ward SQRT is registered in a system table SYSTBL through buses 1', 2', 3' and, after compiling and linkage, the SQRT is stored in a file through buses 4', 5'. When an inquiry command is inputted, the SYSTBL checks th SQRT through buses 1, 2, 3 to analyse the sentence structure. The analysed result is sent to an execution planning part S through a bus 4 and the SQRT is loaded through buses 5, 6. While requesting to execute the SQRT to a user's program execution part I2, an execution part I1 executes inquiry processing through a bus 9.