Abstract: A laser beam-transmitting section of a laser measuring instrument of dimensional change is arranged at one side of a specimen heating furnace, a laser beam-receiving section is arranged at the opposide side of the furnace, a slit fixture for shielding within-the-furnace radiation heat and light is provided inside a measuring window, a heat insulating material having a slit for shielding within-the-furnace radiation heat and light is provided inside said slit fixture, and a within-the-furnace radiation light shielding slit plate and a within-the-furnace radiation light reducing optical filter are disposed at the end faces of said respective laser beam-transmitting section and said laser beam-receiving section whereby measuring errors caused by the light and heat generated within the furnace are prevented (the interior of a specimen heating furnace core pipe can be replaced by various kinds of atmospheres as necessary, and it is possible to measure the dimensional change of small-size specimens of ceramics or t