Medical endoscopes are equipped with wide-angle lenses to provide a wide field of view operating doctors. However, it has been pointed out that there is a backward looking blind area for endoscopes such that an affected area could be overlooked since the gastrointestinal system is intricate and the inside shaped by plicae. In this paper, we propose an omnidirectional vision attachment that has a convex mirror fitted at the tip of an endoscope. The attachment enables backward observations with the endoscope by providing a 360 degrees view. The issue related to developing this attachment is illumination of the field of view. Because the light source is usually only the light at the tip of the endoscope when it is inside the organ, we designed an attachment to illuminate the back-looking viewing field by reflecting light in a mirror. In the experiment, we measured the field of view and the illuminated field, and confirmed that the field behind an endoscope tip can be observed using the attachment.