PURPOSE: To perform write and read-out of history data concurrently by collecting information in each part of a system and recording the collected information in a history memory as history data.

CONSTITUTION: On the assumption that a history memory 12-1 is selected by a write memory selecting circuit 13, history data are successively written in the history memory 12-1. When a prescribed event takes place, the memory selecting circuit 13 selects the succeeding history memory 12-2. As the result, the history data are successively written in the history memory 12-2. When the history data are to be read out from the history memory 12-1, the history memory 12-1 is selected by a read memory selecting circuit 14. After changing the contents of a read address register 15, the whole contents of the history memory 12-1 are read out.