We present a study of the pre-peak phase of the solar flare of M6.4 / 3N class which arose on July 19, 2000 in the NOAA 9087 active region. The effective magnetic field Beff was measured using the FeI 6301.5 Ǻ, FeI 6302.5 Ǻ, Hα and Hβ spectral lines. It was found that at the brightest place of the flare, which was projected onto a small sunspot of N polarity, Beff was close to each other on all four lines and corresponded to 1.0-1.2 kG. At the same time, the modulus of the magnetic field at the level of FeI 6302.5 formation, determined by the splitting of peaks V of the Stokes parameter and the localization of the σ-components in the I ± V profiles, was in the range 1.6–2.6 kG. The bisectors of the I + V and I – V profiles of the FeI 6301.5 line are parallel to each other, indicating a simple one-component structure of the magnetic field at the level of the middle photosphere under the flare. The Balmer decrement of Imax (Hα) / Imax (Hβ) by Hα and Hβ lines was 1.16. The semi-empirical model of the photospheric layers of the flare was constructed using Stokes I observations of non-magneticsensitive FeI 5123.7 and 5434.5 lines by solving the inverse equilibrium transfer problem using Tikhonov stabilizers. For the distribution of temperature with height, the effects of deviation from the LTE were found to be significant for the layers of the lower photosphere corresponding to the heights h ≥ 0 (i.e. τ 5 ≤ 1). In the entire thickness of the photosphere (h = 0–500 km), the flare temperature is lower compared to the non-perturbed atmosphere, while it is slightly higher for h> 500 km. The micro-turbulent velocity is increased at altitudes h> 200–500 km, while at altitudes h <200 km it is reduced. The obtained results indicate that the upper photosphere and the lower chromosphere are perturbed during solar flares, even when the magnetic field is quasi-homogeneous in the lower layers (middle photosphere).