PURPOSE: To enable an apparatus to be used under a poor environment including dust and other pollutants by providing a hot flow sensor for measuring the hot flow flowing through a condensed plane, a temperature sensor or the like for measuring the temperature of the condensed plane.

CONSTITUTION: A thermally conductive body 302 is covered with an insulating layer 303 to protect an apparatus under a severe environment. A temperature changing means for changing the temperature of the body 302 has a cooling coil 308 and electric resistance heating element 304 which are embedded in the body 302. A refrigerant is supplied from a refrigerant source 310 and the heat is monitored by a heat utilizing controller 320. A gas sample enters expansively an air path shielder 307 through a hole 306, and the shielder 307 covers a condensed surface 305. A temperature sensor 301 measures the temperature of the condensed surface 305, and a hot flow sensor 350 measures the hot flow crossing the surface 305. This apparatus is suited for the measurement of dew point in different places in a very large receptacle or the measurement in the field as a portable one.