TOKYO—Suzuki Motor Corp., Japan's fourth biggest auto maker, said 85-year-old Osamu Suzuki, who has been heading the company for nearly four decades, has handed over the title of chief operating officer to his son Toshihiro.

Osamu Suzuki, who married into the auto maker's founding family and become president in 1978, remains chief executive officer, the company said.

Speculation has simmered for years that the elder Mr. Suzuki would name his son as his successor. Osamu Suzuki is one of Japan's oldest active chief executives, frequently attending corporate events and traveling to India.

In recent years, Osamu has been grooming his son Toshihiro, 56 years old, to take over the reins of the company. Trained as an engineer, Toshihiro has been vice president for four years and has been overseeing Suzuki's global marketing operations including those in India.

Under the new COO, Suzuki aims to increase its revenue to ¥ 3.7 trillion ($30.2 billion) by the financial year ending in March 2020, up 23% from the last business year, it said on Tuesday. The company also aims to boost its operating profit margin to 7% from last year's 6%.

The younger Mr. Suzuki is set to take over a niche Japanese auto maker focused on inexpensive small cars and emerging markets. Suzuki's unit Maruti Suzuki is the biggest car maker in India.

He also takes over Suzuki's legal battle with Volkswagen AG.

In 2009, Suzuki and Volkswagen AG entered a partnership, with the German giant purchasing a 19.9% stake in Suzuki for €1.7 billion in 2010. But sharp differences between the two companies' corporate cultures emerged. In late 2011, Suzuki filed for international arbitration, asking Volkswagen to sell back the stake.

A London-based arbitration court is currently reviewing the case.

Write to Yoko Kubota at yoko.kubota@wsj.com

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