Analysis of sub-THz emission from the M 6.4 class solar flare observed on April 2, 2017, by the Bauman Moscow State Technical University radiotelescope at frequencies 93 and 140 GHz is presented. It is found that the spectral flux density of radio emission increases with frequency. Based on the model of solar atmosphere provided by Machado et al. (1980) the contribution function of the radiation source was estimated. It is shown that the observed radiation is formed in a thin chromospheric interlayer with a thickness of about 10 km, in which
the plasma temperature is about 0.1 MK. A homogeneous model of the interlayer is proposed, which makes it possible to explain the observed spectral features. Obtained results shown that the chromospheric flare source is heated by the hot flows generated in the overlying areas.