In December 2005, the winter monsoon was abnormally strong around Japan. Therefore, it was expected that the cooling effect on the upper layer of the ocean would be very strong. However, the mixed layer depth (MLD) in the Kuroshio/Oyashio Extension region was not as deep as those observed during the last four winters. We investigated the reason for the shallow MLD by using a one-dimensional turbulent closure model and the temperature and salinity data obtained using Argo floats. We found that summer heating plays an important role in the determination of the maximum MLD. In 2005, the heat storage rate was very high from July to October since summer heating was significantly stronger than those during the other summers. Consequently, a large vertical density gradient existed at the bottom of the mixed layer and prevented the deepening of the mixed layer.