In this paper, we propose a motion control algorithm for switching between three motion strategies in unknown environment using a gradually building map. It is well known that the motion control of a coverage problem based on a global map is more efficient than local information based motion control which does not use the map. When the beginning of the coverage task, the building map is insufficient, so the motion control based on the map is not so efficient. When almost end of the coverage task, the built map is expected to be almost perfect one. Our algorithm is designed with the insufficiency of the gradually building map. In the algorithm, the robot selects one of three motion strategies depending on completeness of the map. The strategies include local motion control based on local sensing, wall following control to get environmental information and global motion control based on the built map. The robot selects mainly local motion control at an early stage. As time goes on, a global motion control using the reliable map is more selected automatically. The efficiency of the coverage problem is discussed by comparing the proposed method with a control method only by local information. Some simulations and experiments using the algorithm are shown in the paper.