PURPOSE: To reserve latest data and required one at a data memory by deciding the oldest data by every data except reference data and recording the latest data on the recording area of the oldest data.

CONSTITUTION: When a data memory 4 is full and new data is recorded, the reference data is decided by whether a flag that represents the reference data is attached on the data or not. According to the decided result, the oldest data out of sequentially recorded data except the reference data is decided, and the new data is recorded on the recording area of the oldest data.