PURPOSE: To make uniform the temperature in sample region by passing the leg of heating element through a horizontal passage in an insulating material, locating the heating part of heating element in an outer heating region and dividing a plurality of heating elements into two loops in horizontal region.

CONSTITUTION: A heating element 92 has U-shaped leg parts 94, 96 and a curved heating part 98. A central sample region 108 for receiving a sample and an outer heating region 110 for arranging the heating part 98 of element 92 are provided in the center of furnace chamber 62. The leg parts 94, 96 of element 92 are passed through the horizontal passage of an insulation block 82 and a plate 84 and a plurality of elements 92 are arranged horizontally while being divided into two blocks. Since high power can be concentrated into the furnace chamber 62 for heating the sample, temperature can be distributed uniformly along the gauge length of sample.