The Pentagon yesterday took the unusual step of publicly stating that "many in this building believe that the Army's share" of the defense budget "has not been as equitable as it should have been."

The statement, which seemed to serve notice that there will be a different division of the Defense Department's money pie in the future, came at the regular news briefing yesterday in response to questions about a Washington Post report yesterday stating that the Navy might lose as much as $10 billion to the Army over the next five years.

Defense Department executives had said they are searching for money to form and equip five new Army light divisions at the rate of one division a year for the five-year period from fiscal 1985 through 1989.

The Navy's budget, especially shipbuilding, has come under heavy fire from Deputy Defense Secretary Paul Thayer and research chief Richard D. DeLauer in the secret deliberations of the Defense Resources Board, sources said.

In those meetings Thayer and DeLauer are challenging the Navy's assumptions that its aircraft carriers, Aegis cruisers and DDG51 destroyers could survive attacks by the modern cruise missiles and other anti-ship weaponry the Soviets will have in the 1990s, officials said.

The Defense Resources Board is comprised of the top civilian and military leaders of the Defense Department. Thayer has been heading the current round of DRB meetings and has vowed several times to rein in on the Navy in drafting the budgets for the next five years, officials said.

Brig. Gen. Walter C. Cousland, Pentagon spokesman, declined at yesterday's news briefing to discuss the DRB meetings. He said no decisions have been made on how much money each service should receive under the next five-year budget plan now being prepared.

The Army started turning away volunteers at its recruiting stations last year because it has as many soldiers on its payroll as it could afford under the policy decision to stay small while paying for its biggest rearmament program since World War II.

The five new light divisions, which planners envision as mobile enough to fight in the rough mountain terrain of Iran and other Persian Gulf countries if the Soviets should threaten oil fields, will be formed out of existing, heavier divisions, partly to save personnel costs.

Besides the bills falling due for such big ticket items as M1 tanks, new helicopters and a whole family of "smart" weapons, such as antitank missiles, the Army must find money to pay for thousands of trucks and other vehicles on order.

It recently awarded a $1.2 billion contract to AM General Corp. for production of 54,973 High Mobility Multipurpose Wheeled Vehicles, called Hummers. The new vehicle is supposed to combine the abilities of light trucks and Jeeps.

Pentagon analysts are predicting privately that even with a larger slice of the Pentagon budget the Army will not have enough money to pay for all the hardware it intends to buy and therefore must start canceling programs soon. CAPTION: Picture, The Army's High Mobility Multipurpose Wheeld Vehicle called Hummer, for which a $1.2 billion contract recently was awarded. (U.S. Army photo)