The physical conditions for the generation of ion-acoustic waves during linear stage of corresponding instability development in the loop plasma of solar active region before a flare are investigated. It was assumed that instability is caused by the collective action of the effects connected with taking into account the pair Coulomb collisions in plasma, the existence of quasi-static large-scale electric field in the loop current circuit and the adiabatic slow growth of electric field's amplitude. The last-named growth one increases as the magnetic flux interaction intensity increases in the framework of the Heyvaerts — Priest — Rust mechanism of a flare. It is shown that ion-acoustic instability can not appear at the chromospheric part of the loop current circuit during early stage of magnetic flux interaction, when the electron-ion collisions dominate in plasma and the first «forerunners» of a flare, the modified magnetoacoustic or kinetic Alfven waves, appear. This instability can appear much more later, at the later stage of the flare process development, when the anomalous resistance rises in plasma at the investigated part of current circuit.