This data writing device and processing method was invented by Yuki Koike.  This application is based upon and claims the advantage on a previous Japanese patent. A data writing device includes a reader and a processor. The reader receives a radio wave from a wireless tag and measures a radio wave intensity of the radio wave. The processor manages the radio wave intensities of the waves received from the plurality of wireless tags. A reading threshold is set among the radio wave intensities, having the radio wave intensities surpassing the set reading threshold through the reader.