PURPOSE: To sufficiently exhibit performance of a sensor installed in a hood of paper-making drier and simultaneously elongate the life by protecting the sensor from dew condensation.

CONSTITUTION: In a paper machine drier enclosed by a hood 3 and having a drier roll 1 arranged inside, a tube 7 having proper flow path cross section and retained in heat insulating properties by keeping the side wall to double tube structure or stretching a heat insulating material 8 is inserted from the outside into a hood 3 and simultaneously piping 6 for blowing cooling air is connected to the above-mentioned tube 7 to carry out air purge and a sensor 4 is attached near the top of the tube 7.