The aerodynamic characteristics of a straight wing lying on a diameter in a circular open wind tunnel are studied, where minimum induced drag is assumed. In my previous paper, exact expressions of spanwise optimum lift distribution and minimum induced drag have been obtained from Trefftz plane flow field analysis, where an independent variable transformation and Söhngen's inversion integral formula were used. In this study, an alternative analysis is done, where a comformal mapping and Söhngen's formula are used. The results obtained here are coincident with those of Ref. 8.