  The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to
determine the magnitude of the electronic specific-heat coefficient \gamma, in
an attempt to gain an insight into the origin of the unusually large
thermoelectric power of this compound. It is found that \gamma is as large as
48 mJ/molK^2, which is an order of magnitude larger than \gamma of simple
metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated
electron system, where the strong correlation probably comes from the
low-dimensionality and the frustrated spin structure. We discuss how the large
thermopower and its dependence on Ca doping can be understood with the strong
electron correlations.
