  Let $G$ be a finite group and $W$ be a faithful representation of $G$ over
{\bf C}. The group $G$ acts on the field of rational functions $\mathbf C(W)$.
The aim of this paper is to give a description of the unramified cohomology
group of degree 3 of the field of invariant functions $\mathbf C(W)^G$ in terms
of the cohomology of $G$ when $G$ is a group of odd order. This enables us to
give an example of a group for which this field is not rational, although its
unramified Brauer group is trivial.
