1. A method for intracellular metabolic flux analysis comprising the following steps: (a) culturing cells in a medium not containing any isotope-labeled substrate to a target phase of the metabolic flux analysis, (b) adding an isotope-labeled substrate to the medium and collecting a plurality of cell samples from the isotope-labeled substrate containing medium over a period of time, wherein each cell sample is collected at a distinct and different time point over the period of time, (c) measuring isotope distribution of an intracellular metabolite contained in the cells of the plurality of cell samples collected over a period of time, to produce measured data, (d) performing a regression analysis for the measured data comprising calculating to calculate an isotope distribution ratio in a steady state, the regression analysis being performed using a specific function represented by equation (I) 
 MDV ( Mi )ξ={( a*t λ +b )/( d+c*t λ )} η (I); and (e) analyzing a metabolic flux of the cultured cells using the calculated isotope distribution ratio to obtain the analyzed metabolic flux, wherein MDV(Mi) ξ is a mass distribution vector of a substance ξ, η is an integer of for larger, λ is a positive number, t is time, and a, b, c and d are regression parameters providing a minimum nonlinear regression error.