With the aid of the electron microscope, observations were made of changes in the morohological structure of wood fibers as the result of bleaching operation and it was clearly shown that bleaching reaction proceeds topochemically. In addition, the evidence to show that at least a party of the primary wall has been removed to a considerable degree during refining process was confirmed. After stripping the primary wall of wood fibers by brief beating in water in a disintegrator, the fiber bodies were observed and the distribution of the residual lignin and hemicellulose on the surface of the outer lacer of the secondary wall was considered. Further, the swelling behavior of wood fibers in dilute cuprammonium hydroxide was studied. When wood fibers with the primary wall still attached showballoon swelling, the primary wall, having ruptured, appears as an irregularly torn membrane aside the balloon. On the other hand, when same wood fibers are swollen after stripping the primary wall, ballon swelling also arises, but the thin torn membrane is not now seem. The above described evidence supports the idea that the constrictions occurring in the ballon swelling are caused by the outer layer of the secondary wall.