A Clos network is well known as a switching network structure that can achieve a strict-sense non-blocking condition. However, the non-blocking condition is too restrictive to increase the switching capacity of Clos network under the constraint of the limited number of switches. This letter proposes a Clos network design model that guarantees an admissible blocking probability to increase the switching capacity. The proposed design model achieves a larger switching capacity in a practical time than the conventional design model that satisfies the strict-sense non-blocking condition, while guaranteeing an admissible blocking probability.