A storage area setting unit sets (2M+L) storage areas (M>L≥1 and L is an integer) in a storage device, for storing M pieces of data from M data sources (M≥2 and M is an integer). A data specifying unit specifies an L piece of data as write-target data, from among the M pieces of data from the M data sources. A write-destination specifying unit specifies within the (2M+L) storage areas, a storage area that is a write destination of the L piece of write-target data, from among storage areas excluding storage areas from which data is read during the L piece of write-target data is being written, storage areas into which data from each data source has been written last, and storage areas from which data from each data source has been read last. A write processing unit writes the L piece of write-target data into the specified storage area.