Modulation of heat flux DSC can be made by modulated light irradiation to the cells or the cell holders. For this type of temperature-modulated DSC, a theoretical approach has been tried to study steady state and hence its application to heat capacity measurement. In this approach, additional heat flows, such as mutual heat exchange between the sample and the reference material and heat loss to the environment, are taken into account together with heat capacities in the heat paths, which have the effect of amplitude decrement and phase shift of the oscillation. The effect of thermal contact between the cell and the cell holder is also considered. The results are compared with the results for other types of temperature-modulated DSC made by similar approaches to make clear the features of this type of temperature-modulated DSC.