Robust routing schemes are attracting attention as a routing scheme to achieve high robustness against changes in traffic patterns without performing both traffic measurement and dynamic change of traffic routes. This scheme, however, causes the performance degradation of TCP because it performs multipath routing and the out-of-order packet arrivals occur frequently. In this paper, we try to improve the performance by modifying a method (traffic distribution method) that determines how traffic should be distributed among the multiple routes for each source-destination (s-d) router pair that performs multipath routing. In the conventional traffic distribution method, the traffic distribution is determined on a per-packet basis. In this case, the number of packet delivery route changing is proportional to the number of the transferred packets, and consequently the out-of-order packet arrivals occur frequently. In this paper, we propose a per-burst distribution method. In this method, the traffic distribution is determined on a per-burst basis, where a burst is defined as a predetermined number of consecutive transferred packets for each s-d router pair. Numerical examples show that the per-burst distribution method can attain much higher throughput than the per-packet distribution method regardless of the burst length and can keep throughput high even when traffic patterns change if we use an appropriate burst length.