In his relentless drive to modernize South Korea, President Park Chung Hee has insisted in countless speeches that all would march forward together and that no man should take comfort in prosperity while others suffered in poverty.

"It would be most regrettable," he has said, "if there should arise among the people a tendency to ignore others and to engage only in making money for their own selfish interests."

Until very recently, the pledge was being kept to a remarkable degree. Studies by the World Bank and independent analysts marvelled at the prospect: a poor and undeveloped country rapidly modernizing its economy and, at the same time, sharing the fruits in an uncommonly equitable way. South Korea's distribution of income, the studies showed, was among the world's fairest.

But the past few years have soured that optimism. The latest government statistics depict a growing gap between the new middle class and the poor. There is a "puzzling" increase in urban poverty in the 1970s, says one government analysis. The lower 40 percent are getting a declining share of the national income.

Poverty amid riches is a common sight throughout Asia and it is no more striking in South Korea than it is in the Philippines, Malaysia, or Thailand. what is different is the awareness that it was not supposed to happen that way here.

Because almost everyone has os recently experienced poverty, South Koreans have an unusually difficult time accepting the new riches of a relative few, said a government economist.

"The psychological pressure is greater in korea that anywhere else because everyone has been poor," he said. "Everyone knew each other in the old days. We were so very homogeneous then. In other countries, a poor man may never know a rich man. But here they see their friends get rich and they think they should get rich too or something is the matter with them."

In strictly economic terms the widening gulf is a product of Korea's extraordinary success. Its transformation from a low-skill economy of texitle plants and wig factories to one requiring high-skills to turn out cars, ships, electronic equipment, and heavy machinery for export placed a premium on skilled labor, engineers, technocrats and managers. The wages of those people soared.

The unskilled were left behind, laboring for skimpy wages in sweatshop factories at 60 and even 72 hours a week.

One recent government study shows precisely how the gap has grown between the well-educated workers and others. Between 1971 and 1977, the real wages (adjusted for inflation) on those with college degrees rose by 9 percent. The increase was only 2 percent for those of middle school education or less.

Government economists admit they have been surprised by the extent of the wage gap and by the class divisions and social tensions it has provoked. But they insist it will be shortlived. Kim Mahn Je, president of the Korea Development Institute, believes the gap will begin to close as more and more of the unskilled are trained for the abundant higher-skill jobs. "But it will take time to supply these new people," he concedes.

In the meantime, the government rarely intervenes to lift the wages of the poorest paid. It has informally established a wage floor of about $60 a month and warned employers not to pay less than that, although some 2,000 work places still do.

There are no mysteries to the government's low-wage policy. It is deliberately designed to keep Korean exports competitve against threats from other low-wage Asian countries. CAPTION: Picture, Seoul sees itself in severe competition for low-wage industries such as its profitable textile industry.