Kevin Johnson, the executive in charge of Microsoft's struggling online business, is leaving to become chief executive of Juniper Networks.

Johnson, 47, is leaving as his boss, Steven Ballmer, Microsoft's chief executive, is shaking up the senior management ranks in an effort to improve the fortunes of the company's Internet search and advertising business. Microsoft is a distant third behind Google, the leader, and Yahoo in this field.

The departure, announced Wednesday, also comes after Microsoft's failed bid for Yahoo. Johnson played a central role in negotiations with Yahoo.

Under the new plan, responsibility for the two businesses overseen by Johnson - online services and the Windows operating system unit - will be separated.

In its statement, Microsoft acknowledged that it needed new leadership to lift the performance of the online services business. The company said it would "create a new senior lead position and will conduct a search that will span internal and external candidates."

In Microsoft's most recent quarter, which ended on June 30, revenue from online services rose 24 percent from the year-earlier quarter, to $838 million. But the business's loss for the quarter more than doubled to $488 million.

Microsoft's weak performance in online services, analysts note, began well before Johnson took over in September 2005, with the title of president of the platforms and services division.