Mbege is a beer made of banana (Musa spp.) and finger millet (Eleusine coracana). It is the most popular indigenous alcoholic beverage in northeastern Tanzania, and plays an important role in the economy of the region. In this study, we observed and recorded a detailed traditional technique for brewing mbege. We observed that mbege production was divided into three major steps: nyalu preparation, mso preparation, and mixing. Fermented porridge made of ripened banana, called nyalu, was used as a source of yeasts. As a source of fermentable sugars, a sweet porridge made of germinated finger millet called mso was used. In mso preparation, a brewing technique to enhance the effectiveness of saccharification was used. After the preparation, these two types of porridge were mixed. The ethanol concentration of the mixture increased when it was fermented for 6 h, and it then became mbege. It was supposed that yeasts in the nyalu converted fermentable sugars in the mso into ethanol. We found that the brewing technique used in the production of mbege in northeast Tanzania was similar to that used in southern Tanzania. We also demonstrated that the stem bark of Rauvolfia caffra, which was called msesewe and used as an additive in mbege production, accelerated the fermentation of nyalu and therefore increased the rate of ethanol production in the brewing of mbege. This result was consistent with the traditional knowledge in the field about the effect of msesewe on mbege production.