In this paper, a new criterion for the internal transport barrier (ITB) formation is proposed by defining a unique scalar parameter, the profile gain factor. The profile gain factor shows degree of the confinement improvement with respect to an arbitrary reference temperature profile in the L-mode. As the reference L-mode profile for the large helical device (LHD), the edge ion temperature profile data is extrapolated to the core by the L-mode profile function, which is characterized by the thermal diffusion coefficient being proportional to the local ion temperature. The profile gain factor is defined as the ratio of the ion stored energy experimentally measured to that evaluated from the reference L-mode profile. The proposed method is applied to the LHD experimental data, and its capability for quantification of the ITB strength is demonstrated.