How can the Constitution be amended?

Amendments to the Constitution first require the approval of two thirds or more of the members of the House of Representatives and the House of Councilors, and then the approval of the majority of the people of Japan by a national referendum.
Some people feel that the present Constitution was forced on Japan during the postwar occupation, and one can hear voices to the effect that Article 9 should be revised because it should be taken for granted that a modern state should have a military organization to protect its own land, or in order to allow the sending abroad of military personnel to cooperate with peace keeping operations (PKO) of the United Nations.
However, since the enforcement of the present Constitution in 1947, no national referendum for its amendment has been conducted.