To overcome limited maneuverability and procedures in the endoscopic neurosurgery, and to utilize its maximum potential ability, the HUMAN (hyper-utility mechatronic assistant) system has been developed. The system has a rigid three-dimensional endoscope with built-in three guiding-manipulators completely controlled as a slave robot by a surgeon in a remote place. To use this system most efficiently, we investigated the suitable distance between an object lens of the endoscope and a target. Actual working territory (AWT) and actual working angle (AWA) of the forceps in the endoscopical operative field (EOF) were measured by changing the distance between the lens and the target. AWT ratio (AWT/EOF) and AWA ratio (AWA/360) decreased from 58% to 12% and increased from 39% to 85% respectively as the distance became longer. When the distance exceeds 13 mm, AWA did not increase more than 85%, and AWT kept decreasing. For the effective use of the system, the optimal distance between the lens and the target should be within 13 mm, and the system will demonstrate its ability to the maximum at the distance of 8mm.