An assist device is provided with a spring device that generates an auxiliary driving force, by an elastic energy, acting on a joint of a leg. The protrusions enable the imaging element to avoid distortion caused by following the surface of the base, thereby suppressing the effect on electrical properties of the imaging element. Finally, a failure detecting unit detects a liquid discharge failure from data of the scattered light received by the light-receiving element. Each z value is represented within a predefined numerical range which substantially corresponds to a depth range between a near plane and a far plane related to pixel rendering.