The progress of spatial agglomeration of Krugman's core–periphery model is investigated by comparative static analysis of stable equilibria with respect to transport costs. We set forth theoretically possible agglomeration (bifurcation) patterns for a system of cities spread uniformly on a circle. A possible and most likely course predicted is a gradual and successive one, which is called spatial period doubling. For example, eight cities concentrate into four cities and then into two cities en route to the formation of a single city. The existence of this course is ensured by numerical simulation for the model. Such a gradual and successive agglomeration presents a sharp contrast to the agglomeration of two cities, for which spontaneous concentration to a single city is observed in core–periphery models of various kinds. Other bifurcations that do not take place in two cities, such as period tripling, are also observed. The need for study of a system of cities has thus been demonstrated.