The Earth Simulator, a supercomputer operated by a government science agency for evaluating weather and climate change, is the fastest in the world, the Japan Agency for Marine-Earth Science and Technology said.

A Chinese supercomputer for the first time recently took the top spot in the Top 500, a speed ranking for simple calculations, with the fastest Japanese machine ranked fourth. But the Japanese machine won the top position in a ranking of complex calculations.

The Earth Simulator was first in the Top 500 in 2002, but its position has been dropping in the face of an onslaught of foreign supercomputers. Its ranking in this year's Top 500 was 54th. The Japanese supercomputer was fastest in an international ranking for calculating the fast Fourier transform algorithm.

The agency tweaked the supercomputer's system in March last year, allowing it to achieve a computing speed of 12 trillion calculations per second.