In this paper, a method of writing and reproducing high density data with ordinary Audio cassette deck is discribed. In writing, the data N or NRZ code are modulated into PM code to take the positive code N(1) and the negative code N(0) are taken from the complement of the NRZ code, each of which are written into 2 channel track. In reading, the error corrected and the clock pulse can be generated from the reading pulse itself. Also, without modifying the interior circuit of the deck it is possible to use the deck in both the data and audio by adapting a few simple circuits. Over the range of 25HZ-4KHZ, it was possible to write and reproduce at the speed of 787 bps transmission rate.