The present invention realizes a pure object-oriented operating system that allows a flexible system configuration and lowers the number of context-switches. The realization method in the present invention is: when a message sending source object executes a specified method to send a message to a message-destination object, first, execution of the message-sending source object is discontinued and context data corresponding to the message-sending source object is modified to indicate that execution is discontinued. Then, a specified module containing descriptions of message-sending related procedures is invoked and one of said procedures that corresponds to the specified method is executed to retrieve the context data of the message-destination object. Then, the context data found in the above retrieval is modified to indicate that the object corresponding to the context data is under execution, and the message is passed from the message-sending source to the message destination object.