Retail sales showed a surprisingly strong rise of 1/2 percent last month, the Department of Commerce reported yesterday. Not taking into account the effects of inflation, June sales were up 11 percent from the same month last year.

But the Department revised downward the sales figure previously published for May to show a 0.6 percent decline rather than a slight increase. And for the second quarter as a whole, sales slipped by 0.9 percent from the first three months of the year.

This was the poorest showing for the retail sector since the slump in the economy a year ago, Commerce Department official Ted Torda said yesterday. The Reagan administration believes that the economy will be sluggish for the rest of the year.

Despite a drop in new car sales between May and June, sales of all autos -- including used cars and motorcycles -- increased by 1.3 percent during June, the Commerce report showed. This is the second monthly gain in overall sales for the depressed automobile industry.

Durable goods sales last month wre up by 1.3 percent, while nondurables rose 1/2 percent. Sales of furniture, grocery and department store items rose by 2 percent and sales at gasoline stations, clothing stores, drug stores and restaurants were up 1 percent from May, the Commerce report showed.