To provide a method for easily deciding forgery of printed matter, the kind of an ink composition used in printing, or the like, without using an ink composition containing a phosphor or the like.

In this deciding method for printed matter, an ink composition is decided by immersing the decision target printed matter in an alkaline solution of a pH 9-13 to attempt to elute an ink component of the printed matter, by applying chromatography to the solution wherein the elution of the ink component is confirmed based on color change of the solution, etc., to separate and detect (develop) a coloring material in the ink component-eluted solution, and by comparing it with already registered separation/detection (development) data of coloring materials to identify the coloring material.