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 to refer to other tasks relevant to the delegated tasks.