  We propose a new empirical method to estimate the total far-infrared flux of
galaxies from the spectral energy distribution (SED) at wavelengths lambda
shorter than 100 microns. It is difficult to derive the total far-infrared
luminosity from only the IRAS data, though it is one of the most important
properties of galaxies. Observations by Infrared Telescope in Space (IRTS)
indicate that the SED of the diffuse emission from the Galactic plane in this
wavelength region can be derived from the 60 microns to 100 microns color. This
empirical SED relation was improved in order to obtain a better fit to the
Galactic plane data for I(60 microns)/I(100 microns) > 0.6, and applied to 96
IRAS galaxies for which ISOPHOT and KAO data are available at lambda > 100
microns. As a result, the empirical relation describes well the far-infrared
(FIR) SED for a majority of the galaxies. Additionally, the total FIR flux for
lambda longer than 40 microns is derived from the flux densities at 60 and 100
microns by using this model. For the 96 IRAS galaxies, the uncertainty in the
total far-infrared flux of the present method is 26%. The present method is
more accurate than the previous one widely used to derive the total infrared
flux from the IRAS 60 and 100 microns data.
