From an analysis of Stokes parameter profiles measured in a unipolar sunspot, properties of its large-scale magnetic field structure have been derived. With due allowance for our present knowledge about the Wilson effect and the height of line formation, an expression for the field strength distribution H(r, z) has been obtained. The corresponding field strength gradient equals ∂H/∂z = - 1.5 G km−1 at a height z = 300 km, it increases with decreasing z and vice versa. The inclination of the magnetic vector increases from ψ = 0 ° at r = 0 to ψ = 90 ° at the penumbra-photosphere border, its variation across the spot is not consistent with a purely radial (poloidal) field. The field azimuth has been determined across a sunspot diameter on several days with due allowance for the Macaluso-Corbino effect, and significant azimuthal components Hχ have been found especially in the umbra.