03-21U

The behaviour of a three-dimensional magnet with a one-component order parameter near the critical point in the presence of the constant external field is investigated. The high-temperature and low-temperature regions in the vicinity of $T_c$ ($T_c$ - the phase transition temperature in the absence of an external field) are considered. It is shown, that in the small fields the system behaviour is described by the temperature variable only, but in the case of the large fields the role of the temperature variable is not dominant. The respond expressions for the free energy, susceptibility and other characteristics of the system in the vicinity of the critical point are obtained for every of these regions.