Transactions are vital for multi-level secure database management systems(MLS/DBMSs) because concurrent execution of transactions potentially has conflicts among their accessing to shared data. When conflict occurs, only one transaction is granted to access the shared data, other transactions should be delayed until they can safely use the data. Those conflicts may lead to the security problems in MLS/DBMS. If conspirators produce those conflicts intentionally, they can establish the unexpected communication path called covert channel between high security level users and low security level users. This paper proposes a transaction scheduling scheme called Conflict-Insensible Scheduling (CIS) that hides conflicts from low security level transactions to prevent the covert channels.