By John S. McClenahen Japan's economy continues to show signs of improvement. The leading economic index for Japan increased 1% in June, and May's small decline has been revised to a moderate 0.5% increase, says the New York-based Conference Board. "In addition, the strength in the leading index continues to be widespread," says the business research group. Japan's leading index now stands at 98.2 (1990=100). During the first six months of this year, Japan's leading index increased 1.7% and seven of the 10 elements of the index advanced.