A time interval setting unit 220 sets a time interval that depends on the number of items of data stored by a storage unit 240. Then, among the items of data acquired by a data acquisition unit 210, a storage processing unit 230 stores the data in the storage unit 240 at every time interval that is set by the time interval setting unit 220. In this manner, the storage unit 240 retains the data at the time interval that depends on data acquirement time. At this point, a data extraction unit 250 extracts the data by the number of items of data that depends on the number of items of display data, from the items of data stored by the storage unit 240. Accordingly, when the time interval setting unit 220 updates the time interval, it is not necessary to perform subsampling or compression on the data stored by the storage unit 240. In this respect, the data can be retained at the time interval that depends on the data acquirement time with less processing load.
