PURPOSE: To provide practicability of decreasing change of the baseline with abrupt varying environmental temp. by furnishing two joints on the two sides of a replaceable preheat tube of double tube construction, supplying a measuring fluid to an electroconductivity sensor through the inner tubing, and exhausting it from the sheath tubing.

CONSTITUTION: In a device to separate and analyze 14 the object component in a liquid to be measured chromatographically, the electroconductivity is sensed by a sensor 16. Therein the liquid introduced is passed through a joint 2 at the inlet and the inner tubing 1a of a preheat tube 1, and fed from a joint 3 at the outlet to a conductivity sensor 4. The liquid is sent in the reverse direction through the sheath tubing 1b of the preheat tube 1 and exhausted from the joint 2. Accordingly heat exchange is performed between the liquids flowing in the inner tubing 1a and sheath tubing 1b, and the temp. of the influx liquid to the sensor 4 and the temp. of the flowout liquid become the same substantially. Thus the temp. of the influx liquid to the sensor 4 is kept constant substantially, and the baseline of conductivity sensing signals will be stabilized.