A load sensor that is inoperable even when applied with a large load, has a simple structure easily designed to be lightweight and compact and can detect a load with a high accuracy, and a legged robot apparatus including the load sensor in each of legs thereof. The robot apparatus includes the load sensors counting more than one and each of which detects, as an external force, a reaction applied when the leg abuts a ground surface. The load sensor includes a diaphragm, an activating member that is applied with an external force, a driving member forming along with the activating member a double structure and which presses the pressure-sensitive portion, and an elastic member.