We have carried out a precise energy spectral analysis of the superluminal jet source GRS 1915+105 observed with ASCA six times from 1994 to 1999. The source was so bright that most SIS data suffered from event pileup. We have developed a new technique to circumvent the pileup effect, which enabled us to study the spectrum in detail and at high resolution (ΔE/E ≈ 2%). In the energy spectra of 1994 and 1995, resonant absorption lines of Ca XX Kα, Fe XXV Kα, Fe XXVI Kα, as well as blends of the absorption lines of Ni XXVII Kα + Fe XXV Kβ and Ni XXVIII Kα + Fe XXVI Kβ, were observed. Such absorption lines have not been found in other objects, except for iron absorption lines from GRO J1655-40, another superluminal jet source. We carried out a "curve of growth" analysis for the absorption lines and estimated column densities of the absorbing ions. We found that a plasma of moderate temperature (0.1-10 keV) and cosmic abundance cannot account for the observed large equivalent widths. The hydrogen column density of such plasma would be so high that the optical depth of Thomson scattering would be too thick (NH 1024 cm-2). We require either a very high kinetic temperature of the ions (100 keV) or extreme overabundances (100 Z☉). In the former case, the ion column densities have reasonable values of 1017-1018 cm-2. We modeled the absorber as a photoionized disk which envelops the central X-ray source. Using a photoionization calculation code, we constrain physical parameters of the plasma disk, such as the ionization parameter, radius, and density. Estimated parameters were found to be consistent with those of a radiation-driven disk wind. These absorption line features may be peculiar to superluminal jet sources and related to the jet formation mechanism. Alternatively, they may be common characteristics of supercritical edge-on systems.
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