Shippers are about to see a windfall in belly space as carriers start to take advantage of the United States' new aviation agreement with Japan.

United Airlines will more than double its passenger service to Tokyo starting April 5, adding 130 percent more cargo space in the deal. United will increase from six 747-400 passenger flights a week to 14, bringing the airlines' total belly capacity across the Pacific to 280 available tons a week.United also runs DC10-30 freighters to Tokyo six nights a week, carting cargo from Chicago, Los Angeles, and San Francisco to Tokyo, Osaka, Taipei and Manila. The airlines' total freight capacity after next month's increase will be 650 available tons a week.

American Airlines also is doubling capacity, and will continue a partnership with Japan Airlines that will allow it to sell space in the Tokyo carrier's bellies and on its freighters, which will increase from three to five a week out of Atlanta.

''We anticipate we'll go from three daily (passenger) operations to six and will double our capacity in the Asian market by the end of the year,'' said Richard Brannon, American's managing director of alliances. ''From a service standpoint, it gives our customers substantially more access to the Japan market.''

Cargo from the United States to Asia has dropped off since the currency crises began there last year, but the Japanese market remains relatively strong. Japanese demand for American manufactured goods, auto parts and consumer products such as fresh fish has not taken as severe a hit as the traffic to other parts of Asia, according to the Campbell Aviation Group.

All-cargo airlines such as Polar Air Cargo and United Parcel Service also are planning to increase flights and are discussing partnerships with other cargo airlines to expand their services and capacity in the market.