A record/play device includes a recording unit that records, on a recording medium, a series content data sequentially input to the record/play device, a playback unit that plays back, while recording the data by the recording unit, the content data recorded on the recording medium, and a control unit that controls simultaneous record and play. The control unit includes a data size changing unit that changes a data size at which the content data is to be recorded on the recording medium to a data size different from a data size at which content data that has been recorded on the recording medium is recorded.