The diffusion limit of the linear Boltzmann equation with a strong
magnetic field is performed. The giration period of particles around the magnetic field is assumed to be much smaller that the collision relaxation time which is supposed to be much smaller than the macroscopic time. The limiting equation is shown
to be a diffusion equation in the parallel direction while in the
orthogonal direction, the guiding center motion is obtained. The
diffusion constant in the parallel direction is obtained through the
study of a new collision operator obtained by averaging the
original one. Moreover, a correction to the guiding center motion is
derived.