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In this study, we analyze molecular gas formation in neutral atomic hydrogen(HI) clouds using the latest CO data obtained from the four-beam receiversystem on a 45-m telescope (FOREST) unbiased Galactic plane imaging survey withthe Nobeyama 45-m telescope (FUGIN) and HI data taken from the Very Large Array(VLA) Galactic plane survey (VGPS). We applied a dendrogram algorithm to the HIdata cube to identify HI clouds, and we calculated the HI mass and moleculargas mass by summing the CO line intensity within each HI cloud. On the basis ofthe results, we created a catalog of 5,737 identified HI clouds with localstandard of rest (LSR) velocity of $V_{\rm LSR}\le -20$ km s$^{-1}$ in Galacticlongitude and latitude ranges of $20^\circ \le l \le 50^\circ$ and $-1^\circ\le b \le 1^\circ$, respectively. We found that most of the HI clouds aredistributed within a Galactocentric distance of 16 kpc, most of which are inthe Cold Neutral Medium (CNM) phase. In addition, we determined that thehigh-mass end of the mass HI function is well fitted with the power-lawfunction with an index of 2.3. Although two sequences of self-gravitating anddiffuse clouds are expected to appear in the M$_{\rm tot}$-M$_{{\rm H}_2}$diagram according to previous works based on a plane-parallel model, theobservational data show only a single sequence with large scattering withinthese two sequences. This implies that most of the clouds are mixtures of thesetwo types of clouds. Moreover, we suggest the following scenario of moleculargas formation: An HI-dominant cloud evolved with increasing H$_2$ mass along apath of $M_{{\rm H}_2} \propto M_{\rm tot}^2$ by collecting diffuse gas beforereaching and moving along the curves of the two sequences.

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). VII. molecular fraction of HI clouds