Japan reported Tuesday its net overseas assets soared to a record of about $240 billion in 1987, making it the world's largest creditor nation for the third year in a row. Finance Minister Kiichi Miyazawa said the figure was up 33.5 percent from 1986, when Japan's external assets totaled a then-record $180.4 billion. He presented an annual report to the Cabinet Tuesday that said Japan's governmental and private assets abroad at the end of 1987 totaled $1.07 trillion, up 47.3 percent from a year earlier, while liabilities surged 51.9 percent to $830.8 billion. That left Japan with net external assets of about $240 billion. Japan surpassed Britain and other industrialized nations in terms of net external assets in 1985, with a total of $129.8 billion. A Finance Ministry spokesman, speaking on condition of anonymity, said Tuesday the sharp increase came from active investment in U.S. and other foreign securities by Japanese financial institutions and corporate and private investors. The external assets also include private and government loans, trade credits and the diversion of part of Japan's huge trade surplus, which totaled $76 billion last year, to foreign securities acquisitions.