1. A tape recording device for performing operations of position movement, reading, and writing on a tape medium back and forth in a longitudinal direction of the tape medium, and receiving a series of commands from an upper-layer device, the series of commands including mixed read and write commands and giving a plurality of the read commands from a first read command, a second read command,. .. an m-th read command, and an n-th read command, in this order, the tape recording device comprising: a buffer for temporarily storing therein data related to the reading and an additional writing; a tape for recording thereon the data stored in the buffer; a reading and writing head for reading data from the tape into the buffer and writing the data from the buffer onto the tape; control means for reading data from a designated position of the tape and storing the data in the buffer in response to a reading command, and for writing the data stored in the buffer onto the tape from a written data end position thereof in response to an additional writing command; and a non-volatile memory for storing therein data stored in the buffer in response to an additional writing command, wherein the control means further, upon receipt of the additional writing command, stores the data stored in the buffer into the non-volatile memory from a written data end position, returns a completion notification of the additional writing command to the upper-layer device when the data in the buffer is written in the non-volatile memory, and if such a predetermined criterion as to minimize a movement distance by which the tape travels relative to the head is satisfied, moves to a tape writing position where the data held in the non-volatile memory is to be written and additionally writes the data onto the tape from the tape writing position in the course of moving from an end position on the tape of reading data for the m-th read command to a start position on the tape of reading data for the n-th read command after reception of the additional writing command.