PURPOSE: To make it possible to clean a permanently-installed duct while operating ventilating and air-conditioning equipment continuously, to shorten an operation of maintenance and checkup for a long period and to reduce radiation exposure of an operator.

CONSTITUTION: A nozzle 18 for blowing a high-pressure gas is fitted to a permanently-installed duct 17 on the upstream side. A partition 23 is provided in a permanently-installed duct 21 on the downstream side which communicates with the permanently-installed duct 17 on the upstream side, and an inlet-side connecting port 22 and an outlet-side connecting port 24 which connect temporarily-installed ducts 25 and 26 together with the partition 23 used as a boundary are provided. The temporarily-installed ducts 25 and 26 are connected to the inlet-side connecting port 22 and the outlet-side connecting port 24 respectively, and a radioactive material dust collecting device 27 and a blower 28 are connected between these temporarily-installed ducts 25 and 26.