During the Baroque period, fugues were characterized as the apex of long-established contrapuntal technique, and when it changed into a form, Johann Sebastian Bach's fugues created in his later years became the norm. But nowadays even his early fugues are directly ascribed to this tradition (e.g., "permutation fugue" in the New Grove Dictionary). Such an opinion is regarded as a manifestation of "the young master of fugue, as the fruit of his own contemplation" (C. P. E. Bach). However, before 1708, he evidently became skilled in the practice of improvised fugues by organists, rather than the technique of invertible counterpoint based strictly on the north German theory. In this paper, the author reveals that the practice of improvised fugue relied entirely on thoroughbass-based training of organists, and points out the similarities between the practice of improvisation (through the analysis of the Langloz Manuscript) and Bach's creations (through the analysis of his organ fugues [BWV 551 and 566] and permutation fugue [BWV 196/2], which have been highlighted as being influenced by strict invertible counterpoint). Specifically, the countersubject consists of all chord tones, including the 5^<th> and non-harmonic tones such as suspensions, and the harmony changes colorfully as the subject repeats. In addition, the openings of his fugues, which have been especially associated with north German influence, are identical with improvised fugue. On the other hand, the fugue based on strict invertible counterpoint (i.e., the permutation fugue on north German practice) has highly constructed themes that use only consonance, and is constructed from a variety of combinations of these themes. Considering the significance of the technique, north German practice gave weight to "severity", and this severity is not used in Bach's fugues (even his permutation fugue). He had presumably been influenced by north German fugue, but we cannot relate these fugues directly, and have to investigate closely the process of that influence.