Recent plasma confinement experiments in Heliotron-D are described in connection with neoclassical diffusion theory. Weakly and relatively strongly ionized plasmas are produced by coaxial-gun and Joule heating, respectively. In both cases, the observed diffusion coefficients are found to be in good agreement with the theory, under the conditions of weak magnetic field. Diffusion processes in two different magnetic configurations, εt ∼ εh and εt ≪ εh, are also compared experimentally, where εt is the ratio r/R of the distance from the magnetic axis to the major radius, and εh is the modulation depth due to the helical field.