PURPOSE: To obtain an always clear video without clouding a peep show by fitting a clouding preventing means for preventing the peep show from clouding due to the deposition or the like of crystal to the surface of an objective mirror.

CONSTITUTION: The peep show 10 is fitted from the side part of an electric furnace body 1 in the oblique direction. The peep show 10 is formed by connecting an objective mirror body 13 obtained by superposing sapphire rods 11a, 11b, 11c having about 25 in the axial direction and storing these rods 11aW11c in a case body 12 to an eyepiece body 15 obtained by storing a long rod 14 made of quartz in a case body 15a in the axial direction and a ring-like heater 16 is fitted to the mirror surface 13a of an objective mirror body 13 arranged on the upper part of a crucible 4. Even if a material melting solution is to be deposited on the mirror surface 13a of the objective mirror body 13 in the peep show, the solution can not be deposited because the mirror surface part 13a is heated by the heater 16. Thus, an always clear video can be obtained without clouding the mirror surface.