In　order　to　reduce　the　X-ray　dose　in　the　endoscopic　examination　and　therapy,　we　are　developing　an　inside-body　navigation　system　which　can　give　and　visualize　the　direct　3D　position　of　the　tip　and　trace　of　the　endoscope　instead　of　the　conventional　2D　X-ray　fluoroscopy.　In　this　paper,　we　describe　a　method　of　identifying　3D　position　and　direction　of　ultrasound　scanning　probe　inserted　into　inside　of　the　body,　which　gives　endoscopic　images.　We　use　marker　transducers　placed　on　surface　of　the　body,　which　transmit　pulses　synchronized　to　the　ultrasound　scanner.　The　method　of　measuring　position　(direction　and　distance　between　scanning　probe　inside　of　body　and　transducer　outside　of　body)　of　marker　on　the　scanned　image　itself　obtained　with　ultrasound　endoscope　by　using　adaptive　threshold　and　robust　algorithm　is　described.　Further,　we　give　some　results　of　confirmation　experiments,　and　discuss　them.