PURPOSE: To efficiently use a recording medium by changing a time interval to store data in accordance with the difference of a change of time of analog input signal data.

CONSTITUTION: An analog input signal data is passed through a sampling hold circuit and converted into a digital signal data by a digital conversion circuit to store in a storing medium. When the change of time of the analog input signal data is large, the data is digitally stored at a short time interval and, in case of a short time change of the analog input signal data, the data is digitally stored in a long time interval. Simultaneously the data on the passed time and the time intervals are stored.