UPDATE 1-Caisse says CEO Guay on leave until Dec 10

(Adds details on equity portfolio changes)

TORONTO, Nov 20 (Reuters) - The Caisse de depot et placement du Quebec, Canada's largest pension-fund manager, said on Thursday its president and chief executive, Richard Guay, is off work for four weeks on his physician's advice.

Fernand Perreault, executive vice-president, real estate, is leading the Caisse's executive committee and coordinating operations until Guay resumes his duties on Dec. 10, it said in a statement.

The Caisse said Guay informed its chairman of his leave on Nov. 12.

Guay, formerly the organization's chief investment officer, took over as acting president and CEO in May and was formally named to the top posts in September.

In a separate statement, the Caisse said it is converting certain international equity portfolios that were actively managed in Montreal to passive, index-based portfolios, and that 10 employees will lose their jobs.

The investment returns did not justify the "greater effort" of active management, it said. The decision does not affect international equity portfolios managed by external advisers, it added.

The Caisse had net assets of C$155.4 billion ($120 billion) under management at Dec. 31, 2007. It manages investments for various public and private pension plans.

($1=$1.30 Canadian) (Reporting by Lynne Olver; editing by Peter Galloway)