A theory of ordinary and extraordinary wave coupling in the electron cyclotron frequency range in inhomogeneous magnetized plasmas of a toroidal magnetic trap is reconsidered. Reduced wave equations are studied taking into account the poloidal curvature of the magnetic flux surfaces in a tokamak geometry. Based on these equations, the wave field distributions in the vicinity of the coupling region are found in a semi-analytical form. Non-one-dimensional effects of the flux surface curvature on the mode-conversion efficiency in tokamak plasmas are identified and analyzed.