To prevent positional shift of a material to be treated when it is subjected to forced gas cooling after heating while being mounted on a tray by providing a retaining member for preventing or correcting positional shift of the material to be treated (or the container thereof) retractively.

In a vacuum sintering furnace, a heat insulating material 12 is arranged circumferentially on the inside of a furnace shell 11 and carrying rollers 14,... are arranged at the lower part of a treating chamber 13. A tray 21 mounting total five stages of covered boxes 22 is mounted on the carrying roller 14. Retaining members, i.e., frame bodies 51, 61, abut against the left and right side faces of the box 22 mounted on the tray 21. Each frame body 51, 61 is retractable to the left and right through an arm 53, 63 under action of a cylinder 52, 62 fixed to the outside of the furnace. Furthermore, a weight 71 is mounted, as a retaining member, on the upper surface of the box 22 mounted on the tray 21 and the weight 71 can be elevated or lowered through a chain 73 under action of a cylinder 72 fixed to the outside of the furnace.