We have analyzed the structure of the magnetotail current sheet during the northward interplanetary magnetic field (IMF) intervals, using the method devised by Sergeev et al. <1998>. Case studies suggest that systematic variations of the current sheet thickness are seen not only during substorm activities but also during a building up phase of the cold dense plasma sheet when the IMF is northward and the magnetosphere is geomagnetically quiet. It is further found that during such northward IMF intervals the plasma vertical content, namely the product N0λ with the plasma density N0 at the plasma sheet center and the characteristic thickness λ, showed an order-of-magnitude increase within several hours. We estimate plasma transport rate during the northward IMF to be as much as 1026/s, which cannot be neglected in comparison with that during the southward IMF (~1027 s-1).