Аннотация:We consider the main source of far infrared radiation (FIR) in normal spiral galaxies in the IRAS wavelength range 40-120 . There are two competing propositions: (a) the radiation comes from dust heated by young massive stars and (b) there is a second component comparable to or exceeding the first one which is associated with HI-related cool dust heated by a general interstellar radiation field (GISRF) generated by an old stellar disk ("cirrus" component). Here an analysis is given of optical and FIR data and masses of dust and gas for two subsamples of H-detected galaxies separated by their color temperatures of FIR-radiation to compare properties of "cool" and "warm" galaxies. The results give strong evidences in favor of the first proposition claiming that the bulk of FIR is related to star formation even for the subsample of "cool" galaxies. Cirrus radiation should prevail at wavelengths beyond the IRAS range. Differences between "cool" and "warm" galaxies may be accounted for by a different star formation efficiency, which correlates with color temperature of dust.