Recently, the demand of wireless sensor network treating periodical traffic is increasing. This tendency causes the problem of packet collisions among different periodical traffic. Specifically, packet collisions occur continuously in the case that packet generation and forwarding timing of a sensor node overlaps with other nodes. IEEE 802.11 DCF can detect packet collision and retransmit the lost packets to some extent. However, this approach does not solve the collision caused by the overlapping timing, because packets belonging to different traffic collide as long as the transmission timing is similar. In this paper, we propose a transmission scheduling method that shifts packet generation phase of periodical traffic to avoid the overlap of packet generation and forwarding. To realize appropriate phase shift, packet collision event is recorded at each relay node and the information is gathered at the sink. Then, the sink detects the traffic which experiences the most collisions and instructs the corresponding source node to shift the timing of creating packets.