The nation's surplus in merchandise trade in the first half of this year soared 66 percent from a year earlier to 6.57 trillion yen, mainly because of a drop in imports caused by the prolonged economic slump, the Finance Ministry said Wednesday.

The June trade surplus alone expanded 27 percent from the same month last year to 1.22 trillion yen, but the rate of increase is the lowest of the past 15 months of consecutive year-on-year rises, as preliminary figures on a customs clearance basis indicate.

The pace of increase in the trade surplus is expected to slow as economic stimulus measures being taken rev up domestic demand, a ministry official reckoned.