We study the correlation between the spin configuration and the average number of conduction electrons, based on the Kondo lattice type Hamiltonian. We found that a change in the average number of conduction electrons causes a change in the spin configuration. This results from the competition among the strong Coulomb repulsion, Hund's coupling, and antiferromagnetic interaction. A change in spin configuration from an antiferromagnetic ordering to a spin spiral state can be initiated by slightly doping holes into the half-filled state. We also found that the spin spiral state exhibits a correspondingly larger conductance than the antiferromagnetic state. On the basis of these results, we suggest that it is possible to alter the conductance (transport property) of transition metal oxides by inducing a change in the spin configuration through the application of an external voltage. Copyright (C) 2008 John Wiley & Sons, Ltd.