TOKYO—Despite protests from rivals and calls from lawmakers for a closer look at the deal, Japan’s antitrust regulators on Thursday approved a Web-search alliance between Google Inc. and Yahoo Japan Corp.—even as Europe was launching an investigation of Google’s dominance in Internet search and advertising.

The alliance, announced in July, calls for Yahoo Japan, the country’s most-visited Internet portal and search site, to use Google’s search-engine and search-advertising platforms. That will boost the Google engine’s share of searches in Japan to about 90%.

Japan’s Fair Trade Commission said the deal wouldn’t “immediately” violate antitrust rules, explaining it’s limited to a sharing of technology.

“We have not found any evidence that they are collaborating by sharing sensitive information such as ad pricing or any other problematic ways,” said Takujiro Kono, head of the consultation and guidance office in commission’s trade-practices department. The commission said it will monitor the alliance to ensure rules aren’t broken as it develops.

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