North Pacific Intermediate Water (NPIW) spreads in the subtropical North Pacific as a salinity minimum on the not-outcropping surface of 26.8 sigma-theta. However, its formation processes have been unclear yet. I examined its processes with a General Circulation Model (GCM). When GCM is driven by the annual mean wind, a salinity minimum is produced in the North Pacific, as is similar to NPIW except for its lighter density. Being examined with the particle-tracking and T-S diagram, this salinity minimum water is formed by freshening surface flux at the Kuroshio Separation and its density is determined by the surface density there. These processes are inconsistent with observations, thus this salinity minimum should be called ``false NPIW". False NPIW results from the northward extension of the northwestern North Pacific subtropical gyre beyond the Subarctic Front, as is expected from the annual mean wind.Thus this problem will be common to most GCMs and models must be used which can separate the Kuroshio realistically to reproduce NPIW.

In order to avoid the ``false NPIW" problem, GCM is driven by the winter-like North Pacific wind which can separate the Kuroshio around 36N. In this model, a salinity minimum (Modeled-NPIW) appears around 26.8 sigma-theta as is similar to actual NPIW. As examimining Modeled-NPIW formation processes and fully comparing them with observations, NPIW salinity minimum is formed by the Oyashio mixing ratio increasing rapidly, exactly its gradient exceeding ``Critical Gradient", around 26.4--26.7 sigma-theta, and its density is almost determined by the bottom density of this range (then increases to 26.8 sigma-theta by the cabbeling when the Kuroshio/Oyashio mixing). This mixing ratio structure is caused basically by the rapid increase of the Oyashio layer thickness around 26.4--26.8 sigma-theta, resulting from the two factors of the Okhotsk Sea Mode Water (26.8--26.9 sigma-theta) and the winter sea surface density (about 26.5 sigma-theta). The salinity minimum should appear at the less dense layer than the bottom of this rapid increase thickness range by about 0.1 sigma-theta.

Examining the Okhotsk contributions with the isopycnal mixing model, NPIW is formed by its layer thickness increase effect primarily, its salinity reduction effect, however, has smaller contribution as the slight increase of NPIW salinity minimum density. Thus the Sea of Okhotsk is ``indispensable'' for NPIW formation rather than ``important'' due to its low potential vorticity water source.