1. A workflow management system for managing a workflow including a plurality of hierarchically-classified tasks, comprising: a task receiving device configured to receive designation of a task to be delegated; a delegatee receiving device configured to receive designation of a delegatee; an acceptance receiving device configured to receive, from the delegatee, acceptance of a delegation; a first processor that allows a delegator to hide a delegated task and a subordinate task, and allows the delegator to confirm status of the designated task; and a second processor that allows the delegator and the delegatee to refer to other tasks relevant to the delegated task, wherein the delegated task and the subordinate task are not accessible to the delegator after acceptance of the delegation.