The Case method is applied to the critical problem of a slab reactor with finite reflectors. The solution of the albedo problem is used to simplify the analysis of such a two-media problem. The expansion coefficient of the neutron angular flux of the core satisfies two coupled Fredholm integral equations in which some parameters and functions depend on the coefficients of the albedo problem. The approximate critical half-thickness and expansion coefficient are obtained by using the approximate result of the albedo problem.