Recently, deregulation of electric power industry became popular in world wide. In Japan, under partially de-regulated power industry, transmission system is operated by a power company. Although operator of the transmission system tries to determine permissions to connect generators owned by IPPs (Independent Power Producer) and PPSs (Power Producer and Supplier) through considering power congestions for typical power flow conditions, operators inevitably face the congestions of the power system when some generators are in fault conditions. In such a case, operators must resolve the congestions re-scheduling generators owned by their own companies and those owned by IPPs and PPSs. This problem can be solved though a Security Constrained Unit Commitment (SCUC). This paper proposes a new solution algorithm for SCUC by using the Lagrangean decomposition. The method decomposes the SCUC problem into two sub-problems. One sub-problem is a decision problem which determines an operational power flow or generation schedules in which security constraints are taken into account. The other is the unit commitment problem with minimum up/down time constraints for all the units. Several numerical examples are shown to demonstrate the algorithm.