PURPOSE: To realize a novel small highly manipulable light wave alidade, by allowing distance measuring light waves emitted from a light transmission path means to be incident toward a point to be measured while condensing reflected light waves reflected from the aforementioned point to be measured.

CONSTITUTION: An emitting side objective lens system 101 is attached to the inner side wall of a crude oil tank T so that the objective lens thereof is opposed to the liquid surface S of crude oil. Distance measuring light waves emitted from an optical distance measuring device 1 are emitted from the emitting side objective lens system 101 and the reflected light waves are again received by the objective lens system 101 to be returned to the optical distance measuring device 1 through a light wave guide 102. The time delay of the reflected light waves from the liquid surface and distance measuring light waves, for example, phase difference is measured by the optical distance measuring device 1 and the distance between the objective lens system 101 and the liquid surface S of crude oil is precisely measured by said phase difference. By this method, the liquid surface height H of crude oil can be accurately measured.