The electron spin polarization scanning electron microscopy (Spin-SEM) has been used to image the surface magnetic structures of magnetic materials in the order of nra scale. However, the sensitivity of the Spin-SEM so far used is very low. To improve the sensitivity, it is necessary to find the best condition for the electron detection system. The Mott polarimeter has commonly been used in the Spin-SEM. In the present study the performance of the Mott polarimeter is discussed. The geometrical configuration and the value of the post acceleration of secondary electrons to the detector have been experimentally determined or analized by a simple theoretical consideration. On the other hand, in the present study the condition to obtain the highest intensity and the highest signal contrast is seached by using an electron trajectory simulation in the Mott polarimeter. A series of electron scattering events and the electron energy loss in the target of the polarimeter is calculated, and not only the three dimensional scattering trajectory of electrons, but also the spin polarization transfer at every scattering event are traced in the target.