On affine Riemann surfaces

We show that the universal covering space of a connected component of a regular level set of a smooth complex valued function on ${\mathbb{C}}^2$, which is a smooth affine Riemann surface, is ${\mathbb{R}}^2$. This implies that the orbit space of the action of the covering group on ${\mathbb{R}}^2$ is the original affine Riemann surface.