We investigated acceptable forms of mass distribution of the inert dark halo of NGC 3198 by analysing the gravitational instability of the self-gravitating luminous thin disc of this galaxy, for which both the rotation curve and the velocity dispersion curve are well observed. By assuming that the spiral pattern of NGC 3198 represents the gravitationally unstable modes of the galactic disc, we constructed disc-halo models that are consistent with both the observed kinematic data (rotational velocity and velocity dispersion of the disc) and the results of the Fourier pattern analysis of the spiral structure. The main conclusions are: (1) the mass density of the inert dark halo of NGC 3198 is low compared with that of the self-gravitating disc; (2) if a large amount of dark matter exists, it is very extended, much beyond the observed region, and (3) the halo! disc mass ratio inside the last observed point in the 21-cm neutral hydrogen line, at about 30 kpc from the centre, is smaller than 3. These conclusions are derived solely from the dynamical analysis and are free from assumptions about the mass-to4ight ratio. All of these results suggest that the galactic disc of NGC 3198 is consistent with the so-called `maximum disc model'. Key words: galaxies: individual: NGC 3198 - galaxies: kinematics and dynamics - galaxies: spiral - galaxies: structure - dark matter.