Oil production at the world's seventh-biggest oil exporter fell to an average of 1.321 million barrels per day in May from a revised 1.539 barrels in April, NPD said in a statement.
The fall came as a result of planned maintenance at the Brage, Grane, Heidrun, Oseberg and Veslefrikk fields on the Norwegian continental shelf, NDP added.
While oil output exceeded the NPD's forecast by 5 percent, it was 13 percent below the levels seen in May 2013. For the first five months, oil output was almost unchanged from the same period a year ago.
Norwegian gas output in May was down by about 9 percent from the previous month to 8.2 billion cubic metres, NPD said.
Norway is the second-biggest gas supplier to the European Union after Russia.