We consider the diffusion limit of a suitably rescaled model transport equation in a slab with multiplying boundary conditions, as the scaling parameter epsilon tends to zero. We show that, for smooth enough data, the solution converges in the L²-norm for each t > 0, to the solution of a diffusion equation with Robin boundary conditions corresponding to an incoming flux. The derivation of the diffusive limit is based on an asymptotic expansion which is rigorously justified.