Sony Corp.'s plan for its first share sale in 26 years is straining investors' faith in Chief Executive Officer Kazuo Hirai's ability to deliver on growth promises.

The company plans to raise about ¥440 billion ($3.6 billion) by selling common stock and convertible bonds to help finance an increase in production of image sensors used in smartphones, Sony said Tuesday in a statement. The stock, which has doubled in the past year, dropped the most since September after the announcement.

The fundraising comes as Hirai is midway through a turnaround. He has improved profit by cutting costs and generating more revenue from image sensors and PlayStation games, rebuilding confidence in a company that cut its earnings outlook 15 times in the past seven years. Still, investors are concerned about the size and timing of the offerings without more proof it can produce growth.