[en] The experiment of the second harmonic electron cyclotron heating has been carried out on Heliotron DR (R = 90cm, a-bar_p ≅ 7cm) by using a 28GHz, 200kW gyrotron. The plasmas are observed to be heated effectively in a wide range of electron density even above the Ordinary mode cut-off value (overdense plasma). When electron density is lower than the Extraordinary mode cut-off value, electrons are strongly heated and T_eo of ≅ 1.7keV has been achieved at n-bar_e ≅ 0.25 x 10¹³/cm³. The heating is found to be less sensitive to the magnetic field intensity than in larger devices such as Heliotron E and Wendelstein VII-A, and most efficient at B_o (central magnetic field) lower than B_res (resonant magnetic field). These phenomena are discussed from a viewpoint of orbit of electrons accelerated in the RF field with dominant reflections at the vessel wall (cavity mode). The overdense plasmas (n-bar_e > 1 x 10¹³/cm³), are strongly heated at B_o higher than B_res. Their typical parameters are n-bar_e ≅ 2 x 10¹³/cm³, T_eo ≅ 330 eV and the averaged beta value, <β> ≅ 0.4 %. The maximum <β> value is limited by onset of MHD instability to < or approx. 0.5 % which agrees well with a theoretical prediction for the pressure driven ideal interchange instability. A possibility of heating the overdense plasmas through mode conversion processes of injected RF waves into electron Bernstein waves is examined, and it is shown to be plausible in the present experimental condition with multiple wall reflections. (author)