Dioxins, which are already known to exist in the environment, are strongly carcinogenic, and, therefore, it is requested to develop a new analytical instrument for their trace analysis. There are many isomers in dioxins and their toxicities are very different, and, as a result, a selective analytical means is essential for analysis. At the same time, the concentrations of dioxins are generally very low, and it is needed to develop a sensitive analytical means as well. Gas chromatography combined with mass spectrometry is currently used for analysis of dioxins. It is, however, difficult to apply this method to on-line real-time monitoring of dioxins emitting from an incinerator. On the other hand, supersonic jet spectrometry combined with multiphoton ionization/mass spectrometry (SSJ/MPI/MS) is suitable for this purpose, because of its high selectivity given by optically-selective ionization, allowing on-line real-time monitoring of dioxins and their precursors. In this study, we developed SSJ/MPI/MS and used in the detection of chemical species formed in the chemical reaction of dioxin precursors in the presence and absence of a catalyst. It was found that chlorination and dimerization reactions occur efficiently in the presence of FeCl3. Thus, this approach based on SSJ/MPI/MS provides a useful analytical means for continuous monitoring of dioxins and their precursors. We also developed several analytical techniques for the improvement of sensitivity, which include designs of a new type of picosecond dye laser and of ion focusing optics, and a digital counting technique.