TOKYO—For the legions of conservative Japanese bureaucrats who already are sartorially challenged when it comes to casual wear, June 1 will potentially usher in their worst nightmare, as the government introduces a "Super Cool Biz" campaign that advocates wearing Hawaiian shirts, T-shirts and sandals to work as a way to save electricity this summer.

Tokyoites are bracing themselves for a scorching, sweat-filled summer, indoors and out. As the word setsuden, or save electricity,...