A position detection system for an underwater inspection unit or vehicle improves the detection accuracy of the unit's position. The unit includes a pressure sensor for detecting its vertical position, and an inertial sensor for detecting its attitude angle. The unit also includes a range sensor unit for detecting relative distances relative to a neighboring structure A in several directions on substantially the same plane. A control unit computes a position of a detection direction plane of the range sensor unit based on the unit's detected vertical position and attitude angle, and a cross-sectional shape and position of the structure at the computed position of the detection direction plane. Additionally, the control unit computes the unit's relative position with respect to the structure's cross-sectional shape based on detection results by the range sensor unit, and computes the unit's horizontal position based on position information of the structure's cross-sectional shape.