TOKYO—Japanese Prime Minister Yoshihiko Noda, brushing aside mounting popular opposition to nuclear power, said he was determined to restart idled reactors by next summer, adding that it was "impossible" for the country to get by without them or to consider a quick phaseout of nuclear energy.

"If we want to go down to zero, development of alternative energy must be advanced considerably," Japan's new leader said in an interview Tuesday, his first since taking office on Sept. 2. "It's still too early to say if we can get to...