* Seiko Epson soars ahead of its addition to Nikkei on Aug 1
* Steelmakers gain after upbeat earnings
* Fanuc drops after earnings fall short of expectations
* NT ratio at its lowest since April 2016 as small caps outperform
TOKYO, July 31 (Reuters) - Japan's Nikkei ended slightly softer on Monday and at 6-1/2-week lows as a sell-off in index heavyweight stocks SoftBank and Fanuc offset gains in steelmakers and other companies with upbeat earnings.
The Nikkei fell 0.2 percent to 19,925.18 points, its lowest close since June 15. For the month, the Nikkei shed 0.5 percent, snapping a thhree-month winning streak.
Industrial robot maker Fanuc Corp dropped 3.1 percent as its upward revision to its earnings fell short of analysts' forecasts, while SoftBank Group Corp shed 2.3 percent. The two stocks contributed a hefty negative 51 points to the Nikkei benchmark index's fall.
The Topix dropped 0.2 percent to 1,618.61, but eked out a monthly gain of 0.4 percent. Gains in small cap shares helped it outperform the Nikkei this month.
The Nikkei's underperformance led the ratio of Nikkei versus Topix, the so-called NT ratio, to its lowest since April 2016.
Gainers included Seiko Epson Corp, which soared 4.8 percent and was the most traded stock by turnover. The issue will be added to the Nikkei index on Tuesday, replacing Toshiba Corp.
"There were both positive and negative catalysts in the market today, but thanks to strong earnings by some companies, the market's downside was supported," said Takuya Takahashi, a strategist at Daiwa Securities.
Steelmakers gained sharply following solid earnings.
Kobe Steel jumped 8.9 percent and Nippon Steel & Sumitomo Metal gained 3.1 percent.
That helped to make the Tokyo Stock Exchange's iron and steel subindex the best performer among the TSE's 33 industry subindexes, gaining 2.2 percent.
Hitachi surged 5.3 percent after earnings for the April-June quarter beat expectations. Takeda Pharmaceutical gained 3.4 percent on its earnings. (Editing by Lisa Twaronite and Kim Coghill)