PURPOSE: To determine the minute amounts of components such as minute amounts of sulfur, phosphorus and the like taken out from a sample metal as a gas quickly and simply by automatically preparing a coloring test paper during the period, wherein the determining operation of hydride gas is performed.

CONSTITUTION: A piece of coloring-reagent-impregnating test paper, which is contained in a winding reel 25 for the coloring-reagent-impregnating test paper, is guided into a lead acetate container 26 by driving a coloring-test-paper winding device 31. The test paper 24 is impregnated with the lead acetate. The excessive solution is removed from the test paper 24 when the test paper 24 passes through a dryer 28 with a roller 29. In this way, the automatically prepared coloring test paper 30 is made to pass through a reflected-light-intensity measuring device 20 and wound around the winding device 31. When the test paper 30 passes through the measuring device 20, hydrogen sulfide gas is supplied on the upper surface of the test paper 30 through a gas introducing pipe 17 and a sucking nozzle 18. Thus, the coloring is performed. The colored part is lit with a lighting lamp 21. The intensity of the reflected light is detected with a photodiode 22. Then, the concentration of the hydrogen sufide is measured with a coloring strength detector 23, and the content of sulfur in the steel sample is determined.