In distributed file systems used in data center, a well-known congestion collapse called TCP Incast occurs because many servers send data to the same client simultaneously and then many packets overflow the port buffer of the client link. It leads to throughput degradation in the network. In this paper, we propose two methods to avoid Incast which called complete serialization and nearly complete serialization. The first method is a method which completely serializes connection establishments. By the serialization, the number of packets in the port buffer becomes very small, which leads to Incast avoidance. The second method is a method which overlaps the next connection to the current established connection to improve throughput in the first method. Numerical results from simulation with NS2 show the effectiveness of our two proposed methods.