To provide a clinometer with an extremely small diameter (for example, 25 mm or smaller), and a method of measuring the displacement of a drilled hole from a drilling scheduled line using the clinometer.

This clinometer comprises a displacement device 1 displacing in response to tilting, a visual observation device 2 for visually recognizing the displacement device 1, a lighting device 3 for performing irradiation so that the displacement device 1 can be visually recognized by the visual observation device 2, and a casing 5. The casing 5 is constituted in a substantially cylindrical shape, and stores the displacement device 1, visual observation device 2, and lighting device 3 inside. The displacement device 1 comprises a polyhedron 11, and an annular member 12 rotatably supporting the polyhedron 11.