1. An execution order determining device for determining an execution order of concurrently executable tasks, the execution order determining device comprising: a task control information storing section which stores execution conditions under which execution of the respective tasks is started, and an execution order of the tasks; a task executing section which executes the tasks in accordance with the execution order stored in the task control information storing section; a task execution administering section which monitors a state of the task being executed to detect the task whose state is shifted from an execution state indicating that a task is being executed to a process standby state of waiting for a response from another one of the tasks, based on the execution conditions stored in the task control information storing section; and a virtual task administering section which generates a virtual task as a copy of the task whose state is detected to be shifted to the process standby state by the task execution administering section, wherein the task execution administering section responds to a request of executing the another task from the task whose state is shifted to the process standby state, in response to accepting the request from the task whose state is shifted to the process standby state, and the virtual task administering section adds the generated virtual task to the execution order stored in the task control information storing section to concurrently execute the virtual task with the task whose state is shifted to the process standby state.