PURPOSE: To form a capillary to an exact size on a silicon wafer having two kinds of impurities and to make the column portable by etching the p type impurity surface having a specific shape by an electrolytic polishing method using a DC low voltage.

CONSTITUTION: A silicon wafer 12 consisting basically of an n type impurity having crystal orientation <100> is subjected to shallow etching to the p type impurity surface having specific shape and layer having the purpose of forming a column, by which the wafer 12 is worked. A p type high impurity is diffused perpendicularly to the wafer 12 from the front and rear of the wafer and the wafer is deeply and anisotropically etched in an aq. KOH soln. A carrier gas is then injected to the wafer and is discharged therefrom and in succession, a gaseous sample is injected therein within the specified time and a lining agent is formed to the capillary column. The gas is separated as the carrier gas repeats evaporation and absorption. The temp. sensor while such separating gas is kept heated is cooled, by which the gas analysis is accomplished.