Continuing its recent buying spree, Denver-based cable TV giant Tele-Communications Inc. said Sunday it’s taking an 18% stake in the Japanese firm Sumitomo’s Cable Soft Network.TCI unveiled its buy at the kickoff of the 42nd annual National Cable Television Assn. conference here. The event, expected to draw up to 15,000 attendees, runs through Wednesday. TCI international VP Adam Singer declined to place a dollar figure on TCI’s investment, except to say it is a “sufficient” amount of money. Singer said it “makes absolute strategic sense” for TCI to tap into the Japanese market via Sumitomo, one of the world’s largest companies and the biggest cable operator in Japan. Singer says he hopes the new venture “will become a platform” for other program services between TCI and Sumitomo. Sumitomo’s Cable Soft Network serves 400,000 subscribers and offers 20 hours a day of 1970s and 1980s films. Half the pix are U.S. releases, 20% to 25% are Japanese, and the balance comes from other countries, said Yasushige Nishimura, deputy general manager of Sumitomo’s media division. Nishimura said the TCI deal “will certainly ensure the building of CSN into the preeminent basic programming service in Japan.” Sumitomo also holds partial ownership in two other Japanese cable channels: Star Channel, a premium pay movie channel service, and a cable sports channel. For TCI, the Sumitomo deal marks just the latest in a dizzying number of purchases. John Malone, head of TCI, has plunked down millions in the last few months in U S West, a regional Bell operating company; Carolco, the struggling independent film outfit; Sega, a videogame channel; and Interactive Network, a Mountain View-based technology company. There also have been persistent rumors that TCI may soon cut a deal with long-distance telephone giant AT&T.