A procedure called box-scan search which identifies possible weakness in a power distribution network of an LSI is proposed. In the procedure, node pairs having large voltage difference but located in close proximity are considered as good candidates for improving connections using additional wire. The virtual box is the grid space in which the node voltages are compared. Scans of node voltages of the virtual box generate a prioritized list of the fixing point candidate. Experimental results show effectiveness of the proposed procedure for pointing out the node pairs which requires low-impedance connection.