A tilt detecting device comprising, a light source for projecting a pattern, a liquid component having a free liquid surface and for rear-surface reflecting by the free liquid surface, a photodetection element for photodetecting a reflection image of the rear-surface reflected pattern, and an arithmetic processing unit, wherein the arithmetic processing unit is adapted to scan a predetermined range of a photodetection surface of the photodetection element as many times as required and performs averaging processing to a photodetection signal as detected and calculates a barycentric position of the pattern based on signals subjected to averaging processing.