A time interval setting unit sets a time interval that depends on the number of items of data stored by a storage unit. A storage processing unit stores the data in the storage unit at every time interval that is set by the time interval setting unit. The storage unit retains the data at the time interval that depends on data acquirement time. A data extraction unit 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. When the time interval setting unit updates the time interval, it is not necessary to perform subsampling or compression on the data stored by the storage unit and the data can be retained at the time interval that depends on the data acquirement time with less processing load.