While the sign problem of the Dirac fermion is conditioned by the positivity of a determinant, that of the Majorana fermion is conditioned by the positivity of a Pfaffian. We introduce one sufficient condition for the positivity of a Pfaffian. Based on the positivity condition, we study an effective model of the Majorana edge states in condensed matter physics. We also present the application to the Dirac fermion.