A query processing system of a database using multi-operation processing includes a task retrieval unit for retrieving relational algebra as tasks from a query tree, a unit for dividing retrieved tasks into groups for each relation of database, a synthetic relational operation unit for creating a synthetic relational operation by collecting tasks having common sub-expression for the grouped tasks into sub-groups and creating a task of a synthetic relational operation for each sub-group, wherein when there is a plurality of tasks of join operation in the grouped tasks, the synthetic relational operation unit further divides the grouped tasks into sub-groups for each common sub-expression of join operation, collects a plurality of tasks of the join operation into a sub-group, and creates a single task of a synthetic join operation by using logical union to concatenate each task of the join operation collected in the sub-group.