  We discuss the orbital magnetism and the Hall effect in the weak magnetic
field in two dimensional Dirac fermion systems with energy gap. This model is
related to the graphene sheet, organic conductors, and $d$-density wave
superconductors. We found the strong diamagnetism and finite Hall conductivity
even in gapped systems. We also discuss the relation between the weak-magnetic
field formalism and the Landau quantization with the Euler-Maclaurin expansion.
