We investigated some physical conditions of upper-hybrid wave generation at linear stage of development of corresponding instability in the loop plasma of arc structure in active region before a flare. It was assumed that the main reason of rise and development of the instability is the summary action of the effects which take into account pair Coulomb collisions, existence of quasi-static large-scale electric field in current circuit of a loop, and adiabatically slow growth of its amplitude. This last one increases as intensity of interaction of the magnetic fluxes increases in the framework of the Heyvaerts-Priest-Rust theory. The wave perturbations investigated here from the stability point of view, propagate in the direction, which is almost perpendicular to the direction of magnetic field of a loop. In the case under consideration this generated wave is actually the first harmonics of the pure electron Bernstein modes. It is modified by taking into account the collisions and «subdreicer» electric field. It is shown that this wave can appear at the chromospheric, as well as at the «subphotospheric» parts of the current circuit of a loop only at the early stage of the interaction of the fluxes, when the ion-electron collisions dominate in plasma. At later stage of the flare process, when anomalous resistance appears due to the development of the second harmonics instability, the instability of the first harmonics is absent.