1. An external recognition apparatus comprising: a three-dimensional distance measurement device configured to acquire distance information in a three-dimensional space in a predetermined region which is under a hydraulic shovel and which includes a region to be excavated by the hydraulic shovel; a plane surface estimation unit configured to estimate a plane surface in the predetermined region based on the distance information; and an excavation object region recognition unit configured to recognize the region to be excavated in the predetermined region based on the plane surface and the distance information, wherein the plane surface estimation unit estimates the plane surface in the predetermined region by straight line estimation according to a RANSAC method.