On the basis of the so‐called “Fermat's Principle,” that tsunami waves proceed along such a path that the time required is minimum, the differential equation of the path of propagation was derived by applying Calculus of Variations, on the condition that the position of the wave source and that of the observing station are given. Solving the differential equation, for some cases where the distribution of water depth may be expressed by simple mathematical expressions, the path of propagation for each case was determined. The result of the application of the above method to the case of the “Tokachi‐oki Tsunami” in 1968 is introduced as an example in this paper.