A tilt detector comprises a liquid sealing container (2) with a transparent liquid (1) sealed therein to form a free liquid surface (3), a light projecting system (12) for projecting a light beam (13) to the free liquid surface (3) at a given angle so that it is reflected by the free liquid surface (3), and an optical axis dividing means (17) for dividing an optical axis of the light projecting system. The detector further comprises a first light entry means (4) for allowing the light beam (13) from the light projecting system (12) to pass into the sealing container (2), a second light entry means (5) for allowing the light beams (13) divided by the dividing means (17) into the sealing container (2), a first light irradiating means (6) for irradiating the light beam (13) reflected by the free liquid surface (3) from the sealing container (2), and a second light irradiating means (8) arranged face-to-face to the first light irradiating means (6). A light beam (13) entering the liquid sealing container (2) from the first light entry means (4) is irradiated by the first light irradiating means (6), and when the system is rotated and hence turned down by 90 DEG , the light beam (13) divided by the dividing means (17) enters the liquid sealing container (2) from the second entry means (5) and is reflected by the free liquid surface (3) rotated by 90 DEG and is irradiated by the second irradiating means (8). MATH
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