We explain the origin of the infrared continuum in the solar flare of 10 March 2001 which shows a positive contrast in the maximum phase but possibly a negative contrast in the early phase. The general feature is consistent with a flare model heated by an electron beam. By making non-LTE model calculations, we find that in the early phase, when the electron beam bombards an unheated atmosphere, the non-thermal ionization by the beam results in an increased Hopacity which then reduces the emergent intensity. With the flare development, the atmosphere is gradually heated. In particular, radiative backwarming plays a chief role in the heating of the temperature minimum region and upper photosphere. We estimate the temperature rise in these regions for a fully heated atmosphere in which energy balance is attained. In this case, the continuum emission rises above the quiescent value. Therefore, this corresponds to the flare maximum phase. We further find that the energy flux of the electron beam deduced from the hard X-ray emission is large enough to account for the continuum contrast.