HIGASHIDORI, Japan—A gubernatorial candidate promoting more nuclear reactors beat a rival who wanted to freeze them in a quake-hit northern prefecture Sunday, as Japan's troubled nuclear-energy industry faced its first major ballot-box test since the Fukushima Daiichi accident.

Despite the latest disaster, Shingo Mimura, the incumbent governor running for his third term, easily won the election, according to the Japanese media, as he successfully pitched himself as a steady hand who could pull Aomori prefecture's struggling...