A control apparatus, comprising: a buffer for buffering log data sequentially generated; an interface unit configured to be connected with a first external storage device ; a writing unit for sequentially reading the log data buffered to the buffer and sequentially writing the log data to the first external storage device via the interface unit while the first external storage device is connected to the interface unit ; a monitoring unit for monitoring a free space of the buffer ; a warning unit for outputting a warning upon the free space falling below a prescribed value; an internal storage device ; and an external storage device monitoring unit for detecting that the first external storage device is connected to the interface unit , wherein the writing unit is configured to (i) sequentially read the log data stored in the buffer and buffer the log data to the internal storage device while the external storage device monitoring unit detects the first external storage device to be not connected to the interface unit , (ii) write the log data buffered to the internal storage device to a second external storage device newly connected to the interface unit , change the output the destination second external storage device and stop buffering the log data to the internal storage device , while the external storage device monitoring unit detects the second external storage device to be connected to the interface unit . a buffer for buffering log data sequentially generated; an interface unit configured to be connected with a first external storage device ; a writing unit for sequentially reading the log data buffered to the buffer and sequentially writing the log data to the first external storage device via the interface unit while the first external storage device is connected to the interface unit ; a monitoring unit for monitoring a free space of the buffer ; a warning unit for outputting a warning upon the free space falling below a prescribed value; an internal storage device ; and an external storage device monitoring unit for detecting that the first external storage device is connected to the interface unit , wherein the writing unit is configured to (i) sequentially read the log data stored in the buffer and buffer the log data to the internal storage device while the external storage device monitoring unit detects the first external storage device to be not connected to the interface unit , (ii) write the log data buffered to the internal storage device to a second external storage device newly connected to the interface unit , change the output the destination second external storage device and stop buffering the log data to the internal storage device , while the external storage device monitoring unit detects the second external storage device to be connected to the interface unit . (i) sequentially read the log data stored in the buffer and buffer the log data to the internal storage device while the external storage device monitoring unit detects the first external storage device to be not connected to the interface unit , (ii) write the log data buffered to the internal storage device to a second external storage device newly connected to the interface unit , change the output the destination second external storage device and stop buffering the log data to the internal storage device , while the external storage device monitoring unit detects the second external storage device to be connected to the interface unit . <EOS>