In order to perform simultaneous recording and reproduction, a large capacity of buffer memories were required. In addition, it was difficult to record data while reproducing data which was recorded by a different apparatus. The present invention has an objective of providing an information recording medium, a simultaneous recording and reproduction method, and an information recording and reproduction apparatus which guarantee simultaneous recording and reproduction. The simultaneous recording and reproduction is guaranteed by reproducing data recorded in an area having at least a minimum size fulfilling the simultaneous recording and reproduction condition which allows for four access operations, recording data in an area having at least the same size, and performing reproduction and recording alternately by switching reproduction to recording when the data amount in the reproduction buffer becomes full and switching recording to reproduction when the data amount in the recording buffer becomes empty.