Abstract: A melt level or the gap between a melt surface and a heat shield is measured accurately irrespective of how the melt surface is. A laser beam (2) from a range-finding unit (8) is reflected by a scanning mirror (24) and projected on a melt surface (3) through an entrance window (18) and a quartz prism (20) in a chamber (17) of a puller. After specular reflection, the beam forms a measurement spot (31) in the bottom of a heat shield (16) and scatters. Part of the scatter, after specular reflection at the melt surface (3) (secondary reflection), passes through the prism (20), the entrance window (18) and the scanning mirror (24) to the range-finding unit (8). The range-finding unit (8) carries out triangulation using the distance between a laser source and a photodetector therein, and the angle of incidence and the angle of the received laser beam.