In a data processing method of converting (n×m−1)-bit data to multi-level data of m symbols of n bits per symbol, n and m being integers satisfying n≧2 and m≧2, respectively, the m symbols each comprising n-bit data are arranged in m columns in an n×m matrix, with a value of each of the m symbols being set to an even or odd number, by creating the m symbols each including (n−1)-bit data of the (n×m−1)-bit data, arranging the m symbols in the m columns so that the {(n−1)×m}-bit data is arranged in upper-side {(n−1)×m} bits of the n×m matrix, and converting the remaining (m−1)-bit data of the (n×m−1)-bit data to m-bit data so that the m-bit data is arranged in a row in lower-side m bits of the n×m matrix.