The helical magnetic structure of the Mn_0.9Fe_0.1Ge compound under a quasihydrostatic pressure of up to 1 GPa was investigated by small-angle neutron scattering in a wide range of temperatures (5–300 K) and magnetic fields (0–5 T). It is shown that the wave vector of the magnetic spiral increases with pressure. The field-temperature phase diagrams were plotted for a given compound at pressures up to P = 1 GPa. The temperature dependencies of the values of the magnetic fields corresponding to the beginning of the process of the transition of the polycrystalline sample to the conical phase, H_c1, the end of the process of transition to the conical phase, H_c1m, and the transition to the ferromagnetic phase, H_c2, are shown at different pressures. The applied pressure leads to an increase of all the values of critical magnetic fields at low temperatures, which may indicate the stabilization of the magnetic system under the external pressure. This might be caused by the tendency of the magnetic system to be in a commensurate state. Also, the decrease of the magnetic ordering temperature, T_c, with pressure increase is shown. This indicates the approach of the magnetic system to a quantum phase transition to a disordered state with increase of external pressure.