TOKYO, April 1 (Reuters) - Japan's third-biggest copper smelter, Mitsubishi Materials Corp, said on Monday it plans to produce 179,742 tonnes of refined copper during April-September, down 1.4 percent from the same period last year. The company expects output at its Naoshima smelter and refinery in western Japan to fall 2 percent, while production at its Onahama plant, north of Tokyo, will stay nearly flat, it said in a statement. Following are details of the company's output plan, with comparisons against planned production in the second half of the financial 2018/19 year and actual production in the first half of 2018/19.