A moment equation is obtained in the small angle approximation for optical waves propagated through such a random medium that the material parameter fluctuates inhomogeneously in the direction of propagation and homogeneously in the direction transverse to the propagation path. The equation is an extension of the conventional moment equation and is derived by the same method as used in a homogeneous random medium. The limit of applicability of the equation is clear, and the equation can be applied to most of the practical cases of optical propagation through inhomogeneous random media. The inhomogeneous randomness is contained merely within the coefficient of the equation. Some effects of the inhomogeneous randomness are precisely given by solving the moment equation. A multifrequency moment equation can also be given.