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The probability distribution functions (PDF) of density of the ISM ingalactic disks and global star formation rate are discussed. Three-dimensionalhydrodynamic simulations show that the PDFs in globally stable, inhomogeneousISM in galactic disks are well fitted by a single log-normal function over awide density range. The dispersion of the log-normal PDF (LN-PDF) is larger formore gas-rich systems, and whereas the characteristic density of LN-PDF, forwhich the volume fraction becomes the maximum, does not significantly depend onthe initial conditions. %At the characteristic density, thermal pressure andkinetic %pressure due to turbulent motion are comparable, and the material is%statistically stagnated in a turbulent flow. Supposing the galactic ISM ischaracterized by the LN-PDF, we give a global star formation rate (SFR) as afunction of average gas density, a critical local density for star formation,and star formation efficiency. We find that the observed SFR is well-fitted bythe theoretical SFR in a wide range of the global gas density ($10 - 10^4M_\odot$ pc$^{-2}$). Star formation efficiency (SFE) for high density gas ($n >10^3$ cm$^{-3}$) is SFE $= 0.001 - 0.01$ for normal spiral galaxies, and SFE $=0.01 - 0.1$ for starburst galaxies. The LN-PDF and SFR proposed here could beapplicable for modeling star formation on a kpc-scale in galaxies or numericalsimulations of galaxy formation, in which the numerical resolution is not fineenough to describe the local star formation.Comment: 20 pages with 19 figures, ApJ accepte

Density Structure of the Interstellar Medium and the Star Formation Rate in Galactic Disks