PURPOSE: To decrease the exposure to radiation by constituting a sampling rack in such a manner that an ice film is freely formed and thawed on the inside surface of a sampling chamber and drain piping.

CONSTITUTION: The bottom of the sampling chamber 1 and the drain piping 7 are cooled by liquid nitrogen and the ice films are formed on the respective inside surfaces when the liquid nitrogen is supplied via a liquid nitrogen supply source 9 and a filling piping 10 to a liquid nitrogen filling chamber 8 prior to starting of sampling. The ice films are formed on the inside wall surfaces of the respective parts and therefore even if the sample of liquid or sludge, etc. contg. radioactivity is fed from a sample feed pipe 4 in order to make sampling and is passed through the bottom of the chamber 1 and the piping 7 to a waste treating line 17, the sticking and accumulation of the radioactivity on the inside wall surfaces of the chamber 1 and the piping 7 are prevented. Electricity is conducted to a heater 14 via a heater switch 16 to heat and evaporate the liquid nitrogen in the chamber 8 and to heat and thaw the ice films on the inside wall surfaces of the chamber 1 and the piping 7, by which the contamination of the chamber 1 and the piping 7 by the accumulated radioactivity is prevented.