To always clearly grasp a state such as the shape of a sample or the like without receiving the influence of brightness due to respective temperatures by providing a furnace monitor window to which a heat resistant and transmissible member is attached and a window to which a heat resistant and transmissible member is attached at an opposite position with a member to be monitored in a furnace disposed therebetween.

A homochoromatic suppressing window 3 suppresses a homochromatic condition when a sample 13 is monitored from a main monitor window 4. The window 3 is disposed on a furnace wall opposite to the monitor window 4 and has an area larger than that of the window 4. The respective windows 3 and 4 are sealed with heat resistant and transmissible members such as a heat resistant glass in order to prevent inner high temperature atmosphere from being deteriorated or protect a human body and a camera or the like from high temperature heat. Further, for protecting the transmissible member from the high temperature heat and suppressing heat radiation through the transmissible member, the window to which the transmissible member is attached is disposed in a cylindrical member 14 protruding from an experiment furnace main body. The transmissible member is more effectively separated from a high temperature atmosphere.