Using the average action, defined with a continuum analog of the block spin transformation, we demonstrate the presence of a gauge symmetry along the Wilsonian renormalization group flow. As a reflection of this gauge symmetry, the average action satisfies the quantum master equation (QME). We show that the quantum part of the master equation can be naturally understood, once the measure contribution under the BRS transformation is taken into account. Furthermore, an effective BRS transformation acting on macroscopic fields may be defined from the QME. The average action is explicitly evaluated in terms of the saddle point approximation up to one-loop order. It is confirmed that the action satisfies the QME and the flow equation.