TEN YEARS, it is urged, is an excessively and unfairly long prison sentence for Michael R. Milken, the fallen impresario of the junk bond market. It's not as if he had snatched a purse, his defenders say, or had been caught breaking open parking meters or had stuck up a filling station.

That line of argument is as corrupt as Mr. Milken's methods of managing the market. The offenses to which he pleaded guilty were neither trivial nor -- a favorite lawyer's term -- technical. In one count, he acknowledged stealing from the clients of another firm for the benefit of his own. In another, he conceded, he organized a series of fake transactions to enable a client to dodge income taxes. He evaded the rules governing takeovers, and he rigged the prices of stocks.

This country's financial markets are the world's largest, most transparent and most trusted. The country has the strongest possible interest in keeping them that way -- especially now that it is the world's biggest debtor, whose economy depends on its ability to sell enormous amounts of its securities. In some countries' markets, buyers and sellers demand substantial premiums as insurance against the possibility of being cheated by insiders manipulating prices. Because the chances of that have been low in this country, the efficiency of transactions stays high, and traders around the world have confidence that they will be treated fairly here.

If they ever lose that confidence, the consequences will reach far beyond the financial industry. The prosecutors who are pursuing these labyrinthine cases are doing a great public service. Mr. Milken's crimes attacked the fundamental integrity of the American markets on a large scale, and if the judge, Kimba Wood, had treated them lightly, the damage would have been compounded.

This investigation of manipulation and thievery in the financial system is by no means ended. So far each prosecution has produced evidence pointing toward further wrongdoing. It was the speculator Ivan Boesky who provided much of the information leading to the indictment of Mr. Milken and his subsequent pleas of guilty. Mr. Boesky was sentenced to three years in prison, of which he served two. So far Mr. Milken has apparently not provided much help to the prosecutors. If he chooses to cooperate more fully, Judge Wood said, she may reconsider the sentence. Cheating in the financial markets is like rot in a building. The prosecutors have no choice but to keep pressing to determine how far it has gone.