A relative position detecting system of a tubular device includes a shape sensor (10) and relative position detecting means (25). The shape sensor (10) detects a shape of the tubular device (2) to be inserted into a tubular cavity (4) of an object (3). The relative position detecting means (25) detects a relative positional relation between at least one position in a first range in which the shape of the tubular device (2) is detectable by the shape sensor (10) and at least one position in the tubular cavity (4).