Class of the infinitesimal generator of a diffeomorphism in $(\C^m,0)$

Let $F$ be an analytic diffeomorphism in $(\C^m,0)$ tangent to the identity of order $n$. The infinitesimal generator of $F$ is the formal vector field $X$ such that $\Exp X=F$. In this paper we provide an elementary proof of the fact that $X$ belongs to the Gevrey class of order $1/n$.