(Top panel) Continuum-subtracted Hα image of M 101, obtained from http://www.aoc.nrao.edu. The 25th B-band magnitude elliptic contour is shown by a black solid line, along with the position of SN 1909A (marked by a red star) and the center of the galaxy (marked by a red circle). We also show the field of view (FOV) of ALFOSC and the slit position. The width of the slit (1″) has been enhanced in the figure to show, through a color code, the measured metallicity at the position of a bright H ii region that we inspected. (Bottom panel) Metallicity gradient of M 101 from Kennicutt et al. (2003). In the plot we include our measurement, which matches with those that are already published. The linear fit from Kennicutt et al. (2003) is shown by a dashed line. The interpolated metallicity at the SN distance is marked by a red square and its uncertainty corresponds to the fit error. The error bar (±0.2 dex) for our N2 measurement is shown aside. The positions of SN and nucleus are marked by vertical dotted lines. The solar metallicity (Asplund et al. 2009) and the LMC metallicity (Russell & Dopita 1990) are indicated by two horizontal dotted lines.