PURPOSE: To make a worker come in and out a low temperature wind channel as it is low temperature and change a specimen by installing an access tunnel which is connected through with a plenum chamber side part and divided into a low temperature room and a normal temperature room from the chamber side by pressure-resistant doors.

CONSTITUTION: An access tunnel 26 which faces to the side face of a measuring part 2 from the outer part of a heat insulating construction 25 is connected through with a plenum chamber 1 side. The inside of the tunnel 26 is divided into a low temperature room 28 and a normal temperature room 29 from the chamber 1 side by pressure resistant doors 27a-27c and an observation window 30 is installed in the pressure resistant door 27a. A worker opens the pressure resistant door 27c and goes in the normal temperature chamber 29 to take out water which is released from the surface of the body and then opens the pressure resistant door 27b and goes to the low temperature room 29. After it is confirmed that the chamber 1 is not oxygen deficient, the worker opens the pressure resistant door 27a and goes to the chamber 1 and change a specimen 19. In this way, since the worker can go in and out the chamber as it is at low temperature and change the specimen, the time till re-start the experiment can be shortened.