We studied cortical functional connectivity during short-term memory using MEG. We developed a combined method of Beamformer (BF) with Phase Locking Value (PLV) to determine cortical connectivity during a modified version of the visual Stemberg paradigm. The proposed method uses BF to estimate the distribution of the current sources, to which a hierarchical grouping method was applied to detect the regions of interest. Nulling BF further estimated temporal amplitude changes of the detected sources. We determined the phase synchronization between the sources by means of PLV. We applied the method to MEG responses in the alpha-band of 6 participants during the maintenance period, in which participants required to maintain 8 alphabet letters in mind for 2s. We found that the current sources were robustly localized in the bilateral premotor cortices, the bilateral primary motor cortices, the primary somatosensory cortex, the right dorsolateral prefrontal cortex and the right supramarginal gyrus through all trials. Five out of 6 participants showed significant increase in PLV between the bilateral premotor cortices when they answered incorrectly in the later retrieval period. The left and right premotor cortices are considered to compose phonological loop and visuospatial sketchpad of Baddeley's model of working memory, respectively. Our result suggests that functional connectivity increase between the bilateral premotor cortices to maintain our memory when we are difficult to maintain encoded items.