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We have conducted a mapping spectral line survey toward the Galactic giantmolecular cloud W51 in the 3 mm band with the Mopra 22 m telescope in order tostudy an averaged chemical composition of the gas extended over a molecularcloud scale in our Galaxy. We have observed the area of $25' \times 30'$, whichcorresponds to 39 pc $\times$ 47 pc. The frequency ranges of the observationare 85.1 - 101.1 GHz and 107.0 - 114.9 GHz. In the spectrum spatially averagedover the observed area, spectral lines of 12 molecular species and 4 additionalisotopologues are identified. An intensity pattern of the spatially-averagedspectrum is found to be similar to that of the spiral arm in the externalgalaxy M51, indicating that these two sources have similar chemicalcompositions. The observed area has been classified into 5 sub-regionsaccording to the integrated intensity of $^{13}$CO($J=1-0$) ($I_{\rm^{13}CO}$), and contributions of the fluxes of 11 molecular lines from eachsub-region to the averaged spectrum have been evaluated. For most of molecularspecies, 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 involveactive star forming regions. Therefore, the molecular-cloud-scale spectrumobserved in the 3 mm band hardly represents the chemical composition of starforming cores, but mainly represents the chemical composition of an extendedquiescent molecular gas. The present result constitutes a sound base forinterpreting the spectra of external galaxies at a resolution of a molecularcloud scale ($\sim10$ pc) or larger.

Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band