This paper proposes an optimization method of task-allocation for reducing contentions. There have been some attempts for optimizing task-allocation that minimizes the product of the amount of communications and the number of the communication hops. However, since those methods do not consider the occurrence of contentions, the effect has not been sufficient. The method proposed in this paper uses information of concurrent communication to estimate the effect of contentions to find the optimal task-allocation. In three environments examined on the experiments, the proposed method has shown a better effect than the existing method on two environments, tree and fat tree. On these environments, the maximum gain of performance over the existing method was about 25%. On the other hand, on a mesh environment, IBM BlueGene/L, the existing method better effect than the proposed method. As one of the reasons for this, the influence of the packet priority on the network of BlueGene/L to the behavior of the proposed method is discussed.