Position detecting elements are disposed in an endoscope insertion portion at predetermined intervals. Even when the endoscope insertion portion is bent, position data representing the detected positions of the elements is used to infer or detect the shape of the insertion portion. A virtual element is disposed between adjoining elements so that a predetermined condition will be met. Position data of the virtual element is used together with the actually detected position data in order to interpolate data for the purpose of detecting the shape of the insertion portion. Consequently, as if a larger number of elements were disposed, the shape of the insertion portion can be detected highly precisely.