An apparatus has a unit that receives plural messages, which belong to plural transactions and respectively have a pertinent transaction identifier, in an indefinite order; plural processors each having a queue storing the messages belonging to a transaction in charge, and outputting, after the messages belonging to one of the transactions in charge have become complete, the complete messages in the queue to a message processing unit; an allocation unit that outputs the received message to one of the plural processors, wherein one of the processors is identified based on the transaction identifier of the received message and an allocation rule predetermined for the transaction identifier; and a change unit that changes the allocation rule when activating or stopping the processor. When the change unit changes the allocation rule, each of the processors outputs at least messages for which re-allocation is necessary to the allocation unit.