The phase-field method has recently been developed as a powerful tool to simulate and predict the complex microstructure developments in many fields of materials science. In this study, the phase decomposition in the bcc phase of Fe-Cr-Co ternary alloy under an external magnetic field is simulated based on the phase-field method. Since we employed the Gibbs energy of bcc phase in Fe-Cr-Co system obtained from the thermodynamic CALPHAD database of equilibrium phase diagrams, this calculation provides the microstructure changes directly correspond to the phase diagram. The elastic strain energy arisen from the lattice mismatch between precipitate and matrix is evaluated based on the phase-field micromechanics. In order to calculate the magnetic energy which influences the morphological changes of microstructure during phase decomposition, the dipole-dipole interaction energy between the coarse grained magnetic moments is considered, where the magnetization intensity, which is also obtained from the Bohr magneton data in the CALPHAD database, is expressed as a function of composition and temperature. It should be noted that the Bohr magneton data in the CALPHAD database is useful not only for the calculation of the magnetic contribution term in the Gibbs energy, but also evaluating the magnetic energy, especially when it has the composition and temperature dependence. The simulation result for Fe-40at%Cr-40at%Co alloy at 873K shows that the initial nucleated zone with droplet shape has Cr-rich composition. On the other hand, when this alloy is simulated at 913K, the zone composition becomes Co-rich in accordance with the phase diagram. The morphology of microstructure calculated with the external magnetic field demonstrates that the lamella like microstructure elongated along the external magnetic field is appeared with progress of aging. These morphological and temporal developments of the simulated microstructures are in good agreement with the experimental results that have been observed in this alloy system.