In order to clarify the temporal variations of internal ballistics during firing in a cascaded multistage impinging-jet (CAMUI)-type fuel grain, observations of instantaneous flow fields by numerical analysis along with instantaneous grain geometries were conducted. Two static firing tests were conducted under the same conditions, with the exception of firing duration, to obtain the temporal shapes of fuel grains and the characteristics of the regression progress. Two numerical analyses were conducted using the initial and instantaneous geometries to observe internal flow fields. A pair of vortices that is formed near the circumference of the grain due to the change in direction of the flow from the wall jet to the port flow induces a fan-like regression distribution. Two wall jets collide with each other at the center of the grain, roll up and form a fountain-like flow and a pair of gap-scale vortices. These vortices cause an unequal regression distribution on the end faces. On the downstream-end face, the vortices enhance the local regression rate near the axis of the grain. On the forward-end face, in addition to the region near the axis of the grain, the local regression rate at the reattachment points of the vortices increases. These gap-scale vortices disappear as regression progresses because of the dilation of the clearance between the fuel blocks. As a result, the regression rate distributions on both end faces become nearly flat with the progress of fuel regression.