Magnetic tape pre-read method for using the buffer for pre-read efficiently is provided. This method is executed in a pre-read control unit provided with a buffer margin memory portion and a counter. In this method, the size of data block read from a magnetic tape and the contents of the buffer margin memory portion is compared. When the size of the data block is larger than the contents of the buffer margin memory portion, the counter is incremented. If the count of the counter reaches a predetermined value as a result of the increment, the maximum value of the seize of the data blocks read out in the past is set in the buffer margin memory portion. The count of the counter does not reach the predetermined value unless large blocks appear successively. Therefore, even if large data blocks appear sporadically, the contents of the buffer margin memory portion is maintained.