This paper discusses planning problem for a mobile robot with range sensors that detect obstacles around the robot in unknown dynamic environments. In unknown environments, a mobile robot needs to detect obstacles and finds path without collision to go to destination. Around obstacle information are required by motion and mortion are required to collect information. Therefore, the planning problem with observation need to find path to destination and collect information simultaneously. In this paper, path planning problem for a mobile robot with limited field of view sensors in unknown dynamic environments is discussed. The simulation experiments illustrates the planning method proposed in this paper finds motion to look left-and-right motion before across crossing traffic road.