Critical current densities were measured for a bicrystalline $(Sm_{0.8}Dy_{0.2})Ba_2Cu_3O_7$ film under various magnetic fields $({^{ ightarrow_{H}}}_a)$. The fields were varied from -0.7KOe to +0.7KOe for various orientations with respect to the film surface. The curves of the critical current densities $(J_{cb});vs;{^{ ightarrow_{H}}}_a$ showed the well known butterfly-like hysteretic curves. Our data could be analyzed for the two components of field, which are normal and parallel to the film surface, respectively. We combined the effect of $H_perp$ deduced from the data for the normal field $( heta=90^{circ})$ and the effect of $H_parallel$ deduced from the data for the almost parallel field $( heta=2^{circ})$. Our analyses indicate that $(J_{cb})$ depends on the two components of flux density at the grain boundary. All the experimental data for various ${ heta}s$ fit well to this new formula which was obtained by multiplying the factors deduced from the field dependences of these two components.