Dynamics of the domain wall (DW) in ultrathin ferromagnetic films in an external advanced magnetic field h is considered. It is supposed that the dissipation of energy in a magnetic system has viscous character. The DW movement can be divided on two phases. In first phase (precritical), when the magnetic field is smaller than some critical value (h < h(w)), the angle between a magnetic moment and DW plane increases with magnetic field increasing and the velocity of DW v similar to h. At h = h(w) the maximum of velocity is attained, known as critical Walker velocity, and farther is becomes practically independent on the magnetic field. The second phase of DW motion exist in magnetic field h > h(w). This phase is characterized by onset of low-frequency oscillations. Amplitudes of these oscillations vanish at the point h(w) (omega similar to rooth-h(w)). The frequency of oscillation is increasing with field value growing up to h = h(m) = 1.5652 h(w) where it achieves its maximum value. The angle between magnetic moment and DW plane continues to grow and asymptotically tends to pi / 2, i.e. DW tends to become the Neel's type. It is shown that changes in DW motion are closely connected with the peculiarities of the dynamic properties of ferromagnetic systems, where momentum density has restricted value. Accordingly, DW motion, as a whole, exists only under some definite (critical) velocity, where a degree of freedom, associated with forward movement, is saturated. At magnetic fields more than h(w), new (oscillating) degrees of freedom are incorporated. The Goldstone mode, associated with DW forward movement, formally discontinues to exist. As a result of the transformation of former Goldstone mode and appearance of energetic gap, new low-frequency oscillations are generated, localized near DW.