  We have conducted a mapping spectral line survey toward the Galactic giant
molecular cloud W51 in the 3 mm band with the Mopra 22 m telescope in order to
study an averaged chemical composition of the gas extended over a molecular
cloud scale in our Galaxy. We have observed the area of $25' \times 30'$, which
corresponds to 39 pc $\times$ 47 pc. The frequency ranges of the observation
are 85.1 - 101.1 GHz and 107.0 - 114.9 GHz. In the spectrum spatially averaged
over the observed area, spectral lines of 12 molecular species and 4 additional
isotopologues are identified. An intensity pattern of the spatially-averaged
spectrum is found to be similar to that of the spiral arm in the external
galaxy M51, indicating that these two sources have similar chemical
compositions. The observed area has been classified into 5 sub-regions
according to the integrated intensity of $^{13}$CO($J=1-0$) ($I_{\rm
^{13}CO}$), and contributions of the fluxes of 11 molecular lines from each
sub-region to the averaged spectrum have been evaluated. For most of molecular
species, 50 % or more of the flux come from the sub-regions with $I_{\rm
^{13}CO}$ from 25 K km s$^{-1}$ to 100 K km s$^{-1}$, which does not involve
active star forming regions. Therefore, the molecular-cloud-scale spectrum
observed in the 3 mm band hardly represents the chemical composition of star
forming cores, but mainly represents the chemical composition of an extended
quiescent molecular gas. The present result constitutes a sound base for
interpreting the spectra of external galaxies at a resolution of a molecular
cloud scale ($\sim10$ pc) or larger.
