* Nikkei up 11.5 pct so far this month * Fanuc, Honda Motor fall after disappointing earnings guidance * Nomura rises after strong quarterly earnings, sector higher By Dominic Lau TOKYO, April 30 (Reuters) - The Nikkei average fell on Tuesday following weak earnings guidance from Fanuc Corp and Honda Motor, although it remains on track for its best April performance in 20 years on the back of sweeping stimulus measures unveiled this month. The Nikkei slipped 0.4 percent to 13,824.66 by the midday break. The benchmark has risen 11.5 percent this month and is heading for its best April performance since 1993, largely driven by the Bank of Japan's plans to inject $1.4 trillion into the world's third-largest economy in less than two years to revive growth. "One of the biggest takeaway's for the exporters in this earnings season is the conservative FX assumptions they have taken in their guidance for 2013," said Stefan Worrall, director of equity sales at Credit Suisse in Tokyo. "That's probably a bit disappointing. What this underscores is that the BOJ and policymakers still have their work cut out for them to try to convince corporate Japan that the weaker yen is sustainable," he added. Index heavyweight Fanuc Corp slumped 8.4 percent to a three-week low after the industrial robot maker forecast a first-half operating profit of 62 billion yen, representing only 35 percent of the full-year forecast of 175.4 billion yen by 19 analysts polled by Thomson Reuters I/B/E/S. It was the top-weighted loser in the Nikkei, contributing 52 negative points to the index. Honda Motor Co lost 3.2 percent after the automaker's annual operating profit estimate also came in below market consensus. Expectations that Japanese firms would sharply lift their earnings forecasts for the fiscal year ending March 2014 had been high after the yen weakened 22 percent since mid-November, when Shinzo Abe, who became prime minister in December, promised expansionary monetary and fiscal policies to boost the economy. During the same period, the Nikkei has jumped nearly 60 percent. But some analysts say the impact of yen weakness on certain exporters could be less than expected. "In the last five years ... Japanese exporters have been actively trying to diversify their production, reducing their sensitivity to yen strength," a trader said. "In the opposite way, it reduces the benefit to them from the yen weakness," he added. Of the 39 Nikkei companies that have reported quarterly results, 56 percent of them came in below market expectations, according to Thomson Reuters StarMine. That compared with 62 percent in the previous quarter. The broader Topix index gained 0.4 percent to 1,165.85 on Tuesday morning, with volume at 50 percent of its full daily average of the past 90 trading days. NIKKEI RALLY A BOON TO BROKERAGES Nomura Holdings Inc climbed 3.3 percent after Japan's largest brokerage reported its highest quarterly profit in seven years as it cashed in on the surge in domestic shares and booked a one-off gain on the sale of a property affiliate's stocks. It was the most traded stock on the main board by turnover. Rival Daiwa Securities Group Inc rose 5 percent, while the securities sector gained 3.8 percent. Daiwa Securities carries a 12-month forward price-to-earnings ratio of 17.7, below Nomura's 18.6 and the sector's average of 19.2, according to Thomson Reuters Datastream. Lender Sumitomo Mitsui Financial Group advanced 1.3 percent and was the fourth-most traded after Japan's third-largest bank said it expected year-end net profit to rise 52 percent to a record high of 790 billion yen ($8 billion) as a rally in domestic stock prices boosts earnings.