<p>A liquid level height-measuring apparatus comprising a distance-measuring light wave generating means (205) for generating distance-measuring light waves optically modulated at a given modulation frequency; an objective optical means (101) for irradiating the distance-measuring light waves onto a liquid level (S) to be distance-measured and condensing reflected on the liquid level to be distance-measured; a light receiving means (30) for receiving the reflected distance-measuring light waves led through the objective optical means (101); a distance-measuring outward light wave transmission piping means (104) for leading the distance-measuring light waves from the distance-measuring light wave generation means (205) to the objective optical means (101); a distance-measuring return light wave transmission piping means (105) for leading the light waves from the objective optical means (101) to the light receiving means (30); and a reference light wave transmisison piping means (5) having almost the same light path length as the total light path length of the outward (104) and the return light wave transmission piping means (105).</p>
