A desktop thermal lens microscope apparatus can detect a chemical reaction in a micro spatial region within a glass chip, or the like, more conveniently and at any given place. The desktop thermal lens microscope apparatus has small laser light sources such as semiconductor lasers as an excitation light source and a probe light source. Since an excitation light source system (P), a probe light source (Q), and a thermal lens microscope optical system (R, S, T) are integrally provided in a single housing, the size of the desktop thermal lens microscope is as small as that of a desktop microscope.