To eliminate influences of unnecessary reflected light other than a liquid surface in a liquid surface reflection type inclination sensor. In order to attain the object described above, an inclination sensor includes a liquid that forms a liquid surface, an illumination section, a barcode pattern to be illuminated by the illumination section, an optical system that projects the barcode pattern on the liquid surface, a light receiving element that detects the barcode pattern, a storage section that stores a detection image when the liquid surface is removed from an optical path of the optical system, and an arithmetic section that calculates an inclination angle by subtracting the detection image when the liquid surface is removed from a detection image when the liquid surface is present on the optical path of the optical system.