PURPOSE: To enable a fixing operation and a fitting operation for a space holding member to be simplified.

CONSTITUTION: A space holding member (fitting element) 32 arranged at a low temperature refrigerator 1 is comprised of an insertion part 33 inserted into and engaged with an inserted part formed at a flange 28 at a side wall of an inner box at one end thereof, a supporting part (a supporting piece) 34 for holding the flange together with the insertion part, and an extension 37 of L-shaped section extending from the supporting part and abutted against the side wall, and further a fixing part (an abutting wall) 40 abutted against and fixed to the side wall of an outer box at the other end. With such an arrangement as above, it is possible to apply commonly the space holding member 32 and simplify the fitting operation.