We investigate magnetic field effects on the transport through a quantum dot coupled to normal and superconducting leads. Using the modified second-order perturbation theory, we calculate the linear conductance due to the Andreev reflection. It is shown that a maximum structure appearing in the conductance shifts its position with the increase of magnetic field. The detailed analysis of the local density of states and the renormalized parameters elucidates that the maximum of the conductance in magnetic fields clearly characterizes the crossover among the superconducting singlet, the Kondo singlet and the spin-polarized state. (c) 2007 Elsevier B.V. All rights reserved.