[en] Her X-1 was observed during an on-state of the 35-d cycle in May 1983 with the gas scintillation proportional counters on board Tenma. The declining phase of the on-state is characterized by two progressive features: (i) decrease of the main-pulse amplitude, and at the same time (ii) increase of absorption column of relatively cool matter. However, the photoelectric absorption in this cool medium does not explain the feature of the main pulse decrease which occurs at all energies. The spectral shape after the correction for photoelectric absorption is found to remain essentially the same through the main-pulse decrease. This may be interpreted as due to an attenuation of X-ray beam by electron scatterings in a highly ionized plasma at an estimated distance of --3 x 10⁸ cm from the neutron star. In the last part of the on-state, the main pulse was reduced to become invisible and a simple ''sinusoidal'' form was observed. In this period, a steady phase advance of 18+-5 ms h^-1 is detected. The X-ray observed in this period are probably those scattered from the surrounding medium, and the observed phase advance is explained by that the scattering region for the observed X-rays moves opposite to the direction of the neutron star rotation. (author)