* Biggest monthly year-on-year gain in 10 months
* Comes as other retailers continue to suffer weak sales
* Overseas expansion needed to boost share price:fund manager (Adds fund manager comments)
TOKYO, Oct 2 (Reuters) - Japan's Fast Retailing (9983.T) said on Friday that same-store sales at its Uniqlo casual-clothing chain in Japan jumped 31.6 percent from a year earlier in September, marking the biggest monthly gain in 10 months.
The fashion retailer's strong sales came amid prolonged weak consumer spending, which has hit department stores and supermarkets.
The company said the onset of cold weather in the month helped drive up sales of autumn items such as synthetic leather jackets and flannel shirts. A five-day weekend also helped increase customer traffic, it said.
Despite the robust performance, Fast Retailing needs to show the market its growth plan beyond its domestic Uniqlo business to boost its share price from its current range, said Naoki Fujiwara, a fund manager at Shinkin Asset Management.
"Basically, the domestic Uniqlo business is not a growth area," he said. "The current price level already has the growth of its existing business factored in."
Fast Retailing said last month that it aimed to boost annual sales more than seven-fold to 5 trillion yen ($56 billion) by 2020 as it continues to seek acquisitions and overseas expansion.
The company, which is scheduled to announce its full-year results on Thursday, has forecast a 23 percent increase in operating profit for the year ended in August.
During the same period, same-store sales at about 640 Uniqlo outlets in Japan rose 11.3 percent.
Fast Retailing's brisk sales are in stark contrast with other retailers in the country.
Shares of Japan's top department store chains fell sharply on Friday after they reported a decline in September sales, with industry leader Isetan Mitsukoshi's (3099.T) Isetan chain posting a 10.9 percent fall. Isetan Mitsukoshi shares ended down 4.9 percent at 970 yen.
Prior to the announcement, shares of Fast Retailing fell 1 percent to 11,780 yen, outperforming a 2.3 percent decline in the Tokyo stock exchange's retail subindex .IRETL.T. (Reporting by Taiga Uranaka; Editing by Joseph Radford)