The concept of effective demand was once considered as the most crucial one of Keynes's theory. But, this concept was wiped out from the mainstream economics. It disappeared not only from anti-Keynesian New Classical Macroeconomics, but also from pro-Keynesian New Keynesian Economics either. This must be one of the most enigmatic phenomena in the history of 20th century economics. Can someone give a reasonable account of this phenomenon? What kind of lessons can we draw from this history?