The invention provides a tilt angle measuring device, which comprises a discoid container for including a liquid forming a free liquid surface, a light emitting source for allowing a detection light to enter the free liquid surface, a photodetector for receiving the detection light reflected on the free liquid surface and a signal processing unit for detecting a tilt of the free liquid surface based on a detection signal from the photodetector, wherein the signal processing unit sets a detection light storage time of the photodetector so that an error incidence rate of the detection signal is a predetermined value.