The layered cobalt oxides having the CdI/sub 2/-type hexagonal CoO/sub 2/ layer exhibit good thermoelectric properties especially at high temperatures. This is attributed to the fact that large entropy of k/sub B/ln6 is carried by the low-spin-state Co/sup 4+/. Since the entropy of k/sub B/ln6 comes from the spin and orbital degrees of freedom, it is sensitive to external magnetic field. Magnetic field can release the spin degeneracy through the Zeeman effect, and can also release the orbital degeneracy through the high-spin-low-spin transition. Here we report on measurement and analysis on the field dependence of the thermoelectric properties of the Bi-based layered cobalt oxide [(Bi,Pb)/sub 2/Sr/sub 2/O/sub 4/]/sub y/CoO/sub 2/. Magnetic field significantly suppresses the thermopower, resistivity and thermal conductivity. This is consistently understood in terms of the spin-density-wave-like pseudogap suppressed by external fields.