To realize a dense object placement into a container, we propose a robotic packing motion planner by pushing objects to the side of other objects. Our method comprises three planning strategies, i.e., object placement planning, robotic packing-action planning, and action sequence planning. Object placement planning generates objects' placement into a container without gaps between objects. Based on the planned placement, the robotic packing-action planner selectively uses two action strategies where one is to directly place the object in the desired location of a container by using a pick-andplace approach, and the other is to first place the object at a certain distance from the surrounding object and then push it to achieve the placement without gaps. Finally, the action sequence planning plans the order of selected manipulation strategies. Through experiments, we confirmed that the robot efficiently packs multiple objects into a container by effectively using object pushing.