1. A method for controlling a system monitoring device which retains hardware state information of a computer system and OS software state information of a hardware control instruction given by OS software and monitors and controls the entire computer system, the control method of the system monitoring device comprising: storing the information as save information in a non-volatile memory, when state change of the hardware state information and OS software state information is recognized; reading the save information from the non-volatile memory and restoring the corresponding hardware state information and OS software state information before device failure when re-activation accompanying active replacement of the system monitoring device which has failed is recognized; recognizing operating hardware according to the save information and restoring hardware state information generated for the recognized hardware during active replacement; and recognizing operating OS software according to the save information and restoring OS software state information generated for the recognized OS software during active replacement.