In the field of theoretical chemistry, chemical reaction analysis strategies have developed independently: the reaction path analysis, which discusses reaction processes on the basis of the minimum-energy path connecting reactant and product compounds, and the reaction dynamics analysis, which provides dynamical behaviors of atomic movements along simulation time. However, few attempts have been devoted to unifying these methodologies. I have been working to establish a chemical reaction analysis method based on a potential energy surface underlying the reaction path analysis and the reaction dynamics analysis. So far, I have developed the Reaction Space Projection (ReSPer), which enables us to reduce the dimension of a multi-dimensional reaction path network by using the dimensionality reduction method, to construct a low-dimensional potential energy landscape. Furthermore, I have implemented a scheme for projecting dynamical reaction processes obtained by the ab initio molecular dynamics simulation onto the low-dimensional energy landscape and clarified dynamical aspects of chemical reaction mechanisms (as shown in Figure). In recent years, I have applied the ReSPer method to excited-state chemistry to elucidate reaction mechanisms of ultrafast photochemical reactions.