To attain the effective operation of a storage medium library array device.

An array controller connected to plural library units sends a command to each library unit to perform the prescribed processing in response to the command that is given from a host controller, for example, and then decides whether all library units sent the command answer to show that those commands have been properly executed within a prescribed time after the commands are sent. When it is decided that one of library units of an array sent no proper answer within a prescribed time, the array controller decides this library unit as an abnormal one and separates actively this abnormal unit from the array. Thereafter, the controller performs its processing to the commands by means of only the normal library units.