Film detonation propagating in decane-axygen system in a pipe with a vertical-long r~ctangular cross section (16 mm x35 mm) were investigtLted with the steak sch]ieren photograph Inethod by pay ing attention especially to prop~gation velocity of secondary shock waves, An outline of change .of prapagation velocity of th~ secoTldary shack waves is shown and it is snggested that a leading shack wave is a pile of the emaciated secondaly shock waves. In film detonation, therefore, such a process as a shock wave formation with a pile of campression waves occurs continuously, though in gaseous detonation it occurs once for all in transition of combustion into detonation. And further the phenamena in which a secondary shock wave prapagates in company with a combustion front are discussed, and it is shown that these phenomena are similar to gaseous detonation and occur more often when the combustion front is farther Lrom the leading shock wav~. These phe nomena, however, continue for a very short distance, and propagation vslcocity of the secondary shock wave falls down abruptly. On the basis of these facts; it is presumed that a region of gaseous com bustible mixture exists only in a very narrow area directly before a combustion front.