The problem of the magnetic phase separation effect on the magnitude of the magnetocaloric effect is considered. A general thermodynamic generalized Landau theory with a variable particle number is proposed, which allows simple and consistent treatment of the first order phase transition between the magnetically ordered and disordered phases, taking into account the phase separation. The calculation of the magnetic susceptibility and entropy of phases involved in the phase separation was considered. It is shown that the magnetic susceptibility of the magnetically ordered (disordered) phase participating in the phase separation is negative (positive) in the vicinity of the tricritic point, which can result in the inversion of the sign of the magnetocaloric effect.