To provide a method, in which a clearance between a skin plate in the tail section of a shield machine capable of conducting stable measurement without requiring an illuminator and a segment is measured.

Laser beams projected from a laser oscillator 11 are formed in a slitty shape through a lens system 3 while a segment end face section 2b is irradiated with laser beams, the irradiating section is photographed by a camera device 4, the place of light reception corresponding to the quantity of reflected light detected of a specified threshold value or more and the place of the irradiation are detected as distances from a skin plate 1 from the photographed image data, irradiation and photographing to the end face section 2b are conducted while successively displacing the place of the irradiation of laser beams in the thickness direction of the segment 2, and the change point of the place of light reception is detected from these obtained data. The place of irradiation at the change point is regarded as the place of an edge from the internal surface 1a of the skin plate, and the clearance S is measured by subtracting the thickness T of the segment 2 from a distance L up to the place of the edge.