The number $m$ is called the critical number of a finite abelian group $G$, if it is the minimal natural number with the property : for every subset $A$ of $G$ with $|A| \ge m, 0 \notin A$, the set of subset sums $A^*$ of $A$ is equal to $G$. In this paper, we prove the conjecture of G. Diderrich about the value of the critical number of the group $G$, in the case $G=\mathbb {Z}_q$, for sufficiently large $q$.