1. A network system including a primary device and at least one secondary device controlled by the primary device, the primary device comprising: an execution instruction unit for transmitting, when a predetermined instruction condition is fulfilled, to the secondary device an instruction message for ordering an execution of a predetermined process; and an information addition unit for adding, to the instruction message to be transmitted, condition information indicating that the instruction condition is fulfilled, wherein when a response message, which responds that the predetermined process is executed according to the instruction message, is not received from the secondary device within a predetermined period after the instruction message was transmitted, an instruction message to which the same condition information is added is again transmitted to the secondary device, the secondary device comprising: a process execution unit for performing an execution of the predetermined process when an instruction message, which orders the execution of the predetermined process and is accompanied by condition information, is received from the primary device; a history storing unit for storing, in a storage unit, (i) a history of the execution of the predetermined process and (ii) the condition information corresponding to the received instruction message; and a process response unit for (i) transmitting to the primary device a response message after the execution of the predetermined process is performed according to the received instruction message when the condition information accompanying the received instruction message was not stored in the storage unit, and (ii) transmitting to the primary device a response message based on the history of the execution of the predetermined process when the condition information accompanying the received instruction message is already stored in the storage unit.