1. A task management system for executing a task selected from tasks structuring a task combination, comprising: a judging unit for judging whether there is a task combination switchover request or not; a switchover unit for switching over the task combination after completing an execution of a specified task in the tasks structuring the task combination before the switchover when judging that there is the switchover request; and an execution unit for executing the task selected from the tasks structuring the task combination switched over by said switchover unit.