To save the labor of an operator while reducing input mistakes by simplifying the settings of a switching program for automatically changing over the flow of a sample gas.

A chromatogram that is a result of a first monitoring analysis carried out only by a combination of a first column 4 and a first detector 8 is displayed on the screen of a display part 24, and the start point and completion point of a period during which the operator want to carry out an analysis more in detail by the mouse operation of an input part 23 are indicated on a chromatogram. Upon receiving this indication, a central control unit 22 not only automatically creates a switching program demarcating the period defined by the start point and the completion point to display the switching program, but also alters the color of a chromatogram curve corresponding to the period to facilitate visual discrimination against another part.