The present invention provides a NOx-concentration measuring apparatus D
for quantitatively analyzing the concentration of NOx contained in a
sample gas. The measuring apparatus D comprises a sampling probe for
obtaining the sample gas, a drain separator 2 for condensing moisture
contained in the sample gas as a condensed water and separating the
condensed water from the sample gas, an NO.sub.2 converter 3 for
converting NO.sub.2 contained in the sample gas into NO, a secondary
cooling device 7 for additionally cooling the sample gas, and an NO
analyzer 1, arranged in this order with respect to a sample-gas line of
the NOx-concentration measuring apparatus. The drain separator is a
high-flow-velocity cooling type drain separator.
Further, the sample-gas line between the sampling probe and the drain
separator is heated and/or thermally insulated over the entire length
thereof. The measuring apparatus can provide a high-precision measurement
while suppressing NO.sub.2 loss.
