It is shown theoretically how much the magnetic field influences the collision frequencies between charged and neutral particles. The magnetic fieldBfor the usual field strength does not affect the collision cross section. However, the respective charged particles gyrate around the field lines, and then the relative velocity between charged and neutral particles changes withB. Consequently, it is shown that, when the mean free path &lgr; is large compared with 2&pgr;rL, the velocity component contribution to the collision is the velocity component due to the motion along the field axis and not that due to the gyration. Here,rLis a Larmor radius.